I've been thinking about this a lot and have been studying every source we have on The Great War, how did the Dominion manage to achieve such success against the Empire in a relatively short time frame? Here is my understanding of the history from Oblivion to Skyrim, and why the Empire suffered so grievously during the war. If I am incorrect on any count, please let me know, especially if a lore book is involved.

I'm sure my spell checker screwed me over on more than a few words. I'll try to correct any incorrectly replaced or misspelt words, but thank you for putting up with my terrible grammar and writing skills.


The aftermath of the Oblivion crisis. When the Oblivion crisis ended, so too did the 3rd era and the Septim dynasty. For the next 17 years the Empire was without a leader or a working government and suffering from brutal infighting and political intrigue until Titus Mede I assumed control of the Imperial City and claimed the title of Emperor in 4E 17. During this time of Imperial infighting the events of the Red Year transpired in Morrowind when the Ministry of Truth became unstable and crashed into Vivec city with all of its original force and destroyed the city. This force is believed to have caused Red Mountain to erupt and thus taking the entire island of Vvardenfell with it. Shortly after the event the new rulers of Black Marsh, the An-Xileel, who had declared their independence from the Empire during the crisis invaded Southern Morrowind's now desolated and depopulated countryside, taking several key cities before retreating for unknown reasons to an expanded Black Marsh border. It is unknown what happened during this time apart from the outcome, did the Empire defend Morrowind, was it even able to, or did they persuade the An-Xileel to a lessened conquest and thus was the cause of them halting their advance and retreat? No one knows, records about this war are non-existent.

Rise of the Thalmor During the same period the Summerset Isles saw the resurgence of the Thalmor as a dominate player. Contrary to popular belief the Thalmor did not suddenly appear or vanish from the Summerset Isles. The Thalmor had in fact been a powerful (minority) political force throughout the history of the Summerset Isles after the fall of the first Dominion. It was due to the Oblivion crisis that they gained a far more dominate foothold in the Altermi government with the Crystal Tower being forced by circumstances to cede one right away after another to the far more militaristic Thalmor as the Daedric hoards overwhelmed the land. When the Crisis had ended, the Thalmor stood as the dominate political force in the Summerset isles and began to forcefully solidify their position by eliminating opposition and rivals. By 4E 22 the Thalmor's control was complete, and they renamed the Summerset Isles to Alinor.

The Aldmeri Dominion and the road to war between 4E 22 and 4E 28 the Thalmor instigated anti-Imperial agitation within Valenwood, and in 4E 29 the Thalmor succeeded in staging a coup with the help of Thalmor operatives and Thalmor allied extremists inside the Valenwood government. The very day of the coup a Thalmor invasion force landed on the shores of Valenwood and obliterated the Imperial garrisons located within its border and quickly drove the Valenwood government and Bosmer Imperial supporters to neighboring Cyrodiil and Elsweyr. Less than a year later the Thalmor formally incorporated Valenwood into its new Empire and rename itself the Aldmeri Dominion with the Thalmor at its head. Meanwhile the Empire, still paralyzed by internal governmental fighting despite the new Emperor was powerless to act upon the aggression.

For the next 70 years the Aldmeri Dominion instated an iron curtain between itself and its borders, severing all communications. The Empire meanwhile regains stability and starts the long process of rebuilding itself after the Oblivion crisis and the Red Year. In 4E 98 the two moons over Nirn vanished from the night sky and caused great panics throughout the known world, but especially in Esyswer where the moons are held in great cultural and religious importance. However, by 4E 100 the moons are restored, but not before the Dominion claimed to have used Dawn Magic to restore them shortly before. It is unknown whether the Dominion actually told the truth about restoring them, or if they had found out when the moons would return and simply took credit for it, but regardless this was more than enough to sway enough of the Elsweyr government into staging a coup in 4E 115 and ousting the Imperial legions whose influence had diminished radically after the announcement and subsequent restoration. Imminently after the coup the Aldmeri Dominion proclaimed the former Imperial province as a client of their own, granting them military protection from a forming Imperial Legion to the North.

Sometime between 4E 120 and 4E 168 the Imperial province of Hammerfell fell into a state of civil war over succession between the Crowns and forebears, a state of civil war that continued throughout the Great War. Tension came to boiling point when Titus Mede II ascended the throne in 4E 168, prompting the ambassador from the Dominion to issue his an ultimatum on the 30th of Frostfall, 4E 171 to step down from power, disband the Blades, outlaw Talos worship, cede large portions of Cyrodiil and Hammerfell to the Dominion, and pay staggering tributes, and when the Empire refused the Dominion unleashed its forces upon it who had already been crossing the border before the demand was officially given.



The Great War According to Legate Justianus Quintius the Dominion launched a massive surprise attack against the Empire and caught the entire legion in Cyrodiil and Hammerfell off guard, destroying much of it. The Dominion flanked Leyawiin and Bravil, laying siege to the cities while mauling the Imperial forces stationed there who desperately fought to escape entrapment. At the time Lady Arannelya leads a 2nd force past Anvil and Kvatch and invades South-Eastern Hammerfell while a third Aldmeri army lands in South-Western Hammerfell. Despite the overwhelming invasion, Hammerfell refuses to unify and offers only token local defenses against the Aldmeri onslaught, leaving the enormously outnumbered Imperial force to redeploy to the North to avoid encirclement. Within a month the entire Southern coast of Hammerfell falls to the Dominion, excluding the city of Hegathe which fights a lasting and bitter siege with the help of Crown and Imperial forces that could not, or would not escape north.

By 4E 172 Bravil, Leyawiin, and Anvil have all fallen to the Dominion, whilst Kvatch continues to hold out (Anyone who looks at the land around Kvatch in Oblivion would know how they could). The Aldmeri forces now advance to the Imperial City itself, only to engage the Imperial navy in a string of conflicts that manage to hold open an area that allow the city to be supplied. By 4E 173 Imperial reinforcements from Skyrim, Morrowind, and Highrock supplied Hammerfell and Cyrodiil with desperately needed resistance as Hegathe and Kvatch continues to withstand the assaults and abuse thrown at it by the Almeri armies beside them. However, nothing further is known about the fate of Kvatch, it can only be assumed it eventually succumbed to the siege.

Despite the Imperial reinforcements, the Dominion crossed the Niben and by 4E 174 the Imperial city was surrounded on three sides, with only the battered Imperial fleet keeping the Northern Niben clear to allow supplies from Bruma in. The situation for the Empire was bleak in Cyrodiil, however in Hammerfell rays of home started to shine. A Forbear army from Sentinel broke the siege of Hegathe while the Imperial legions advanced south and reclaimed much of Southern Hammerfell, but despite massive losses to the Imperial and Aldmeri force, a stalemate ensured in Hammerfell as the Forbears and Crowns unified and began to pool their resources and recruit manpower in Hammerfell once again.

It was at this time that Ulfric Stormcloak was captured by the Dominion, and like all Aldmeri prisoners he was brutalized and prayed for information. According to the 'book' entitled Thalmor Dossier: Ulfric Stormcloak upon learning of his status as son of the Jarl of Windhelm, he was given priority for interrogation by Elenwen. Despite what many people believe, Ulfric did not divulge information until after the Imperial city was sacked, despite Elenwen claiming contrary for purposes unknown. He was then allowed to escape according to Elenwen for reasons unknown; however this may just be her attempt to cover her failures. During his imprisonment in the spring of 4E 174 Aldmeri reinforcements massed outside the Imperial city and the final push began, finally resulting in the sacking of the Imperial city several weeks later. Emperor Titus Mede II fought his way out of the city at the head of his men, but at the cost of the entire compliment of the 8th Legion who stood as a rear guard and fought to the last man upon the Northern walls of the Imperial city. What followed was the burning of the Imperial city, the desecration of its crypts, temples to Talos, and the looting of the White-Gold tower itself. In addition, genocide and other atrocities against the civilian populous took place, similar to perhaps the fates of the other captured Imperial cities. This monstrous example of Thalmor barbarism proceeded until The Battle of the Red Ring a year later.

After the disaster the Legions in Hammerfell under the command of General Decianus received word to rally back to Cyrodiil, despite the success they had achieved in Hammerfell. Decianus, unwilling to let his men's sacrifices go to waste issued an order to allow the early retirement of many of his legionaries in exchange for services rendered onto the Hammerfell militias. These same men are credited as the decisive force who inflicted such greavis loses upon Lady Arannelya's force during the battle of Skaven, that she was forced into a rout across the desert where she and her force where obliterated by the waiting Alik'r raiders.


During the Winter of 4E 174/175 the Empire officially dispatched diplomats to arrange a peace treaty. The Dominion believing itself to be secure halted their advance to consolidate their gains in Cyrodiil and continued persecuting the population. However, this was a ploy by Emperor Titus Mede II who led his reformed legions into what would become known as The Battle of the Red Ring and serves as an example of classic Imperial doctrine and strategy. The Empire split its forces into three sections, General Decianus commander of the legions in Hammerfell would lay in wait in the Colovian highlands, eventually sweeping down upon the Aldmeri forces from Chorrol. General Jonna, commander of the Nordish legion prepared his men around Cheydinhal, while the main body would follow the Emperor South from Bruma and advance upon the Imperial City. When the word was given, all three forces advanced out from their perspective staging points and launched a series of encirclements that annihilated much of the Aldmeri forces in the area. Aldmeri reinforcements from Bravil and Skingrad attempted a counter attack, but the Nordic legions repulsed them as General Decianus's legion swept in, taking Skingrad and Bravil thus surrounding the Imperial City and all Aldmeri forces in the province within 5 days of the operation's beginning.

Almost imminently after their encirclement the Aldmeri commander, Lord Naarifin, issued an order to sally forth and attempt to escape the Imperial City. However, much to his dismay the sheildwall of Jonna's Nordish legionaries halted them and he was soon forced to retire. It was at this point did Titus Mede II lead his men over the Northern walls of the Imperial city wielding my Goldbrand and defeated Lord Naarifin himself in personal combat, forcing him to submit to Imperial capture. His body was then put on display high above the devestated city, hung from the White-Gold tower itself. The complete obliteration of Aldmeri forces in the Imperial provinces had been complete, yet came at a terrible cost. None of the 5 Imperial legions left had more than 50% of their original manpower able to fight, while three entire Imperial legions had been destroyed during the war. To make matters worse, entire Southern provinces of Hammerfell and Cyrodiil lay in ruin and the land unable to support its own populations, yet alone the needs of the combined Imperial legion.


I'll post the next segment in the 2nd post, the forums don't like big posts and are giving me trouble.

The White-Gold Concordat Despite the Empire’s newly found position, it was in no position to continue the war. Legate Justianus Quintius believed that the Empire’s reasoning for accepting the terms had been “However, there is a great difference between agreeing to such terms under the mere threat of war, and agreeing to them at the end of a long and destructive war. No part of the Empire would have accepted these terms in 4E 171, dictated by the Thalmor at swords-point. Titus II would have faced civil war. By 4E 175, most of the Empire welcomed peace at almost any price.” Titus Mede II believed the current position was going to give the Empire the best possible peace terms, and believed the Dominion had been preparing a 2^nd wave, particularly aimed at Hammerfell.

Instead of waiting for the Empire’s situation to worsen, he authorized a peace deal that saw diminished, yet still brutal Aldmeri demands. These demands included the outlawing of Talos worship, dismantling of the Blades, and the cession of Southern Hammerfell to the Dominion. The Empire begrudgingly signed the insulting terms believing it was in agreement with the rest of its provinces. However, Hammerfell refused to sign the terms and in order to maintain the peace treaty the Empire was forced to renounce ownership of the province- much to the Redguards ire.
Legate Justianus Quintius notes that this achieved exactly what the Aldmeri had originally indented, sowing lasting bitterness between the Empire and Hammerfell, making them easy prey for conquest. Despite official Imperial proclamations, the majority of the Imperial legionaries who had been under General Decianus stayed in Hammerfell under the command of their most recent commanders.


Battle of Hammerfell Despite the devastation to Southern Hammerfell, Northern Hammerfell was left relatively untouched, and due to the immobile forces of the Crowns and Forebears during the initial stages of the war, Hammerfell had more untapped population centers to recruit from. The Aldmeri on the other hand had been left in a devastated state with the lion’s share of its army destroyed during the battle of the Red Ring and whose navy could scarcely support an invasion force after the battles of the Nibon. Further, the Dominion needed to leave men opposite from the Empire to mount a defense in the event they attacked, placing further restraints on their forces in Hammerfell. Despite the valor of the armies of Hammerfell, the Dominion quickly succeeded in capturing Southern Hammerfell and solidifying a foothold and line of supply. This however was planned by the Redguards who wanted to lure them across the vast deserts of Hammerfell where they would become exhausted and vulnerable to Akri’r raids as they had been during The Great War. Over the next five years the Dominion’s forces attempted time and again to cross the deserts, only to be repulsed by the valiant warriors of Hammerfell and their unofficial Imperial volunteers from . Unable to maintain its position indefinitely, the Aldmeri agreed to sign the Second Treaty of Stros M'kai in 4E 180 that left Hammerfell intact and unbound to the White-Gold Concordat.


Conclusions The Aldmeri Dominion achieved a dominate footing early on in the war by using the elements of surprise and divisions within the Empire. Their navy proved effective and their tacticians insightful, while their strategists thought long term. The Empire on the otherhand suffered a series of catastrophic blows that weakened them and the failure of the Blades to detect the massive Thalmor buildup along the border proved devastating. The Empire was also slow to mobilize its strength, particularly in Skyrim and Highrock where it took years to organize and muster sufficient numbers. The Thalmor favored attacks from within the Empire, very reminiscent to the civil war in Skyrim- suspiciously so even to set the stage for their conquests. Many in Hammerfell claim that their victory proved that the Empire made a terrible mistake in signing the White-Gold Concordat, however historian Legate Justianus Quintius argues that their victory was only made possible by Imperial sacrifices during The Great War and the pinning of Aldmeri armies in Valenwood and Elsweyr which prevented the Dominion from brining their entire force to Hammerfell, although in my opinion given the status of their fleet this would not have been possible anyways.

Reserved space, i'll look at the Markarth incident and the events leading up to the civil war after a bit more research. I'm anxious to hear what others found out, and any possible points that i've misinterpreted or misread.


The Forsworn rebellion In 4E 174 the Forsworn, a people who originally inhabited Markarth before the time of the Nord, sensing an opportunity with the ongoing hostilities of the Great War marched upon the city of Markarth and sized the city and hold in short order. For two years the Forsworn held dominion over the lands as the Jarls of Skyrim and Empire had been locked in bitter combat against the Dominion, yet contrary to much of what is said about the occupation it was a relatively peaceful occupation. The Forsworn occasionally executed and or dealt other forms of retribution against landowners and the upper class nobility in the hold, but by many accounts of the populace the Forsworn ruled fairly when dispensing justice. For the duration of the occupation the Forsworn had consistently requested recognition as an independent province and status as an Imperial protectorate, yet negotiations stagnated until the end of the Great war in 4E 175.

When The Great War ended, Imperial envoys to Markarth entered meaningful descriptively peaceful negotiations according to the book entitled 'The Bear of Markarth' with a surprisingly peaceful rule. However, despite the negotiations agitation amongst many Nords-particularly the now escaped Ulfric Stormcloak who had already been approached by Sigmund, son of the deposed Jarl of Markarth who promised Ulfric many concessions in exchange for retaking the city. Ulfric was not the only one growing weary of the negotiations, the Imperials too had been mustering a force, all be it slowly, with the intention of assaulting the city or pressuring the Forsworn depending on the progression of diplomacy until that point. The Imperials reasoning for such a slow gathering of soldiers had been two fold, by far the most important had been the necessity of prioritizing the Imperial legions in Cyrodiil to quickly replenish their loses from The Great War that had just got over and the tensions over Hammerfell. The second reason being the fact that every capable legionary around Markarth had been replaced by local unskilled militia and dispatched to General Decianu's legion, who inturn released most of them in 4E 174 to continue fighting inside Hammerfell under local armies. The force the Empire had available at the time had been nothing short of local peasents who had been given a uniform and told to maintain public order. The Empire needed to wait until the appointed changing of the guard to avoid attracting Forsworn superstitions about legionary activity.




The Markarth incident By 4E 176 Ulfric had gathered a sufficient force and attacked Markarth. The only details we have of the battle itself come from pro-Ulfric civil war time sources, so the testaments may or may not be inflated and glorified. However, according to those sources Ulfric led his men from the front, using the Thu'um to force open the gate and drive the Forsworn from the walls. Ulfric and his militia made short work of resistance within the city itself, and very quickly established a superior position with few loses. The Forsworn realizing that with their walls breached and their forces outnumbered that their situation was hopeless. Not long after the had attack begun did the Forsworn officaly surrendered to Ulfric and lowered their banners and weapons around the city. However, what followed was a brutal and ugly incident that in my opinion is pretty difficult to justify if sources prove accurate.

Imminently after Ulfric took control of the city, he and his military began a prolonged period of retribution, anyone even accused of collaborating with the Forsworn was executed. He systematically tortured and details imply the [censored] every Forsworn who surrendered for information on the rest of their forces throughout the Reach, even bands who had nothing to do with the attack on the city. His men didn't even spare the native Nord population, and a great portion of the population of Markarth was given the same treatment. Everyone who did not join Ulfric on his march through the city during the battle was hunted down, labeled a traitor, and executed in similar style alongside the Forsworn, even children where not spared. Did Ulfric issue the order for the incident to happen? Perhaps and perhaps not, however, as the commander of the force it is impossible to deny that he was unaware of what was taking place at the time.

Many Stormcloaks claim the Markarth incident was simply Imperial propaganda, an attempt to demonize Ulfric's role as heroic savior. While it is true that many of the official documentations concerning the incident come from Imperial scholars, they are also written prior to 4E 201 and before the official start of the civil war. In my opinion it is not unlikely to suggest that both sides inflated their perspective for wartime purposes and the truth of the incident will remain lost to circumstance, there is overwhelming proof that something terrible indeed happened in Markarth, yet at the same time the entire event seems unspoken in Markarth itself upon visiting the city. To what extent are the records true? Perhaps it was indeed factually accurate, but perhaps not. The main authority on the incident can hardly be called pro-Ulfric, indeed the Bear of Markarth gives an anti-Ulfric pitch near the end of his book, although he also damns the Empire in the same breath. I doubt we will ever truly know for sure.


When the newly formed Imperial legion arrived at the city they where initially denied entry on the premise that Ulfric had demands that had to be met first. An unknown amount of time passed (hinted at several days) before the unnamed legionary commander (Possibly Jonas as he commanded the Nordish legion) accepted Ulfric's terms and established marshal law and assumed military administration. In exchange, they allowed the open worship of Talos as par Ulfric's demands for handing over rule of the city even though this was a violation of the White-Gold Concordat.

Keep in mind that the rest of the leadup to the events of Skyrim are marred in controversy given the Civil War that is birthed from it. I've tried my best to remain unbiased despite my strong views on the issues, but I am only a Human and it's likely that I may miss several points or drifted into pro-Stormcloak/ Pro Empire lore sources that may color the events.

The birth of the Stormcloaks The partnership between Ulfric Stormcloak and Igmund came to an abrupt end when the Thalmor became aware of the legalization and open worship of Talos by the short lived Imperial military administration ruling over Markarth. Jarl Igmund, sensing the impending danger allowed Thalmor agents inside the city where they ambushed and arrested Ulfric and his men, and brought them to an unspecified location. The exact date of the incident is not recorded in any text, however it can be speculated that it happened sometime around 4E 177-178 when Igmund was given back rulership of Markarth and would lijkely have had that power.
It’s not know how long Ulfric and his men had been kept in captivity (three months to six years), but this caused enormous friction between the Empire and the Dominion's already fragile peace treaty. It was during this time that the Jarl of Windhelm died, leaving his son Ulfric Stormcloak as his successor, worsening the situation as the Thalmor had now been holding one of the most powerful Jarls of Skyrim in captivity. Eventually the Empire secured Ulfrics release, but the scene he returned to in Markarth was not at all like what he had left.

The Thalmor agents had ruthlessly purged the city of Talos worship, executing any and all suspected of Talos worship while the Imperial legion had long since been recalled elsewhere, presumably Cyrodiil. From Ulfric’s point of view the situation must have seemed insulting. Ulfric had originally formed his militia and assaulted the Reach at the behest of Igmund under the condition Talos worship would be reinstated, yet under pressure from the Thalmor, Jarl Igmund betrayed Ulfric to captivity. Ulfric also brokered an agreement with the Imperial legion’s commander that in exchange for entry into Markarth, he would enforce the legalization of Talos worship, an agreement that clearly died upon Thalmor arrival.

In retrospect of the situation, the Imperial General likely did not have the authority to accept such a demand, while Ulfric was playing with fire the entire time. Regardless of any potential blame on any party, this event marked the birth of the Stormcloak movement, with Ulfric's militia becoming (presumably) the first of the Stormcloaks.

Potente Ocato was steadily repairing the Empire and holding it together, the Thalmor then had him assassinated thus plunging the Empire into further disrepute. In my opinion this ensured that the Empire was weak and without a capable leader and so unable to hold together leading to the ceding of Elsweyr and Black Marsh. All this made it easier for the Aldmeri Dominion to gain power over the Empire in a shorter space of time.

[source: UESP]

Good find, I had no idea the Thalmor did that.

Good read. As a historian I tend to like this sort of stuff lol. I'll add a few of my points.

First, while the Empire did seem in some chaos after the death in Martin it wasn't as bad as most people seem to assume. Ocato was named Potentate and seemed well on his way to restoring stability. The Hist and Argonians were in revolt but, according to Rising Threat, it seemed that the Empire still had troops there and was fighting to put down the rebellion. Even though the Thalmor were beoming more powerful I don't think they had enough power at this time to make an overt move to seize power while the Empire watched.

This all changed with the eruption of Red Mountain in 4E 6. This catastrophe would be of immense help to the Argonians in finally braking free of the Empire. Ocato did his best to restore stability but he would be assassinated in 4E 10. His death led to seven years of civil war in the Empire and most likely gave the Thalmor the opening they were looking for to seize power. It's only in 4E 10 that the Thalmor gain civil power. Despite that it does seem like the Thalmor didn't take complete control of Summerset Isles until around 4E 22.

You may also be interested in the novels The Infernal City and Lord of Souls (it's really one book split into two) which take place around 30 years after the Oblivion crisis.

What I want to know, how much manpower does the Aldmeri Dominion have? They are very xenophobic, they live for a long time, which means it takes long time before they reach advlthood. Also, I've read somewhere that Altmer are very selective of their offspring and often don't keep weak children. This means that their soldiers aren't as easily replenished as the Empire's.

Altmer do use goblins in the armies and I assume they multiply much faster than men, but still...

I still dont see the the Thalmor succeeding in the future. The way I see it is...

If Empire losses Skyrim:
-Thalmor will start the war again; driving their way back into Cyordiil, setting up a new assault on Hammerfell/High Rock, and working their way toward Morrowind and Black Marsh.
-Due to their movement eastward they will undoubtedly go to war with the Argonians and the Dunmers. (Both don't recognize the Aldmeri panthenon)
-While the Empire will be effectively destroyed, much of Skyrim, Hammerfell, and High Rock will likely have massive amounts of rebellion against the Thalmor.

If the Empire retains Skyrim:
-Much of the Legion can be pulled out of the area to buff their borders.
-They might lose the Reach
-Hammerfell will work with (not rejoin) the Empire in an attempt to continue to push back the Thalmor
-The Argonians might start an offensive against the Thalmor in the south seeing them as a potential threat


Or at least thats what I think

There is no way the Thalmor have the manpower to attack all of these other provinces again. Unless they strike a deal with the Falmer...
If anything the Thalmor would attack each province one at a time saving Blackmarsh for last since they are the equivalent to China.

The make cheap toys?

jk.

I've read some that due to the Great War the Thalmor suffered heavy losses, making them unable to do any overt military action on their part. Thalmor are covertly manipulating all conflict to their benefit, at least that's what it looks like to me.

It's a very real possibility, it's easily suggestible that they suffer just as terrible loses as the Empire did during the war, perhaps more so given the 5 years spent in Hammerfell after the official end of The Great War.

A little bit spoiler-y for people who haven't done the whole Thalmor embassy quest. You know, the Ulfric dossier bit.
However, I'm enjoying reading.
(or am I in the spoilers allowed area?)

Definitely referring back to this. Ultimate thread on everything to do with the Dominion and Great War, (and all without being crazily rabid like the Imperial vs Stormcloak threads tend to get). Really good work. Very comprehensive.

Thanks, though I've bound to have made a number of errors in my sequencing of time.

In what way?

Lots of population I think, although admittedly I've never been to China...are they lizard-men? If so i'd be genuinely surprised to learn that.

Good read

They have crappy products?

Very nice. I've always been very interested in TES lore, so I'm glad there's someone to do the work for me

That is according to the "Bear of Markarth" a biased Imperial Book. According to dialogue found in-game it was Igmund who exacted the retributions. The Forsworn says it was the Jarl (Igmund) who took revenge upon the Forsworn and their families for their murder of his father.
Basically what happened was that whilst the Empire was licking it's wounds Igmund asks Ulfric to take Markarth back. Ulfric does so in exchange for Talos Worship and Ulfric keeps up to his words but Igmund under the pressure of the Empire doesn't keep his end of the bargain and let's the Imperials arrest Ulfric. Then Igmund and the Silver-Bloods arrest the Forsworn and massacre their remaining soldiers along with their families whilst using the higher-tier of the Forsworn as pawns in the Reach's political world.

I said many of the sources are from Stormcloak and Imperial accounts, specifically naming The Bear of Markarth, and said that any mention of the incident, pro or against, are strangely absent in Markarth itself. I also stated that the Stormcloaks claim that the entire history is simply propaganda and that Ulfric was nothing short of a hero. In my opinion this was deliberately done to add a shrowl of mystery to the entire incident and further grey out the civil war and its justifications. The alternative would be to simply omit the entire event on the premise that its records are disputed, but in doing so we would cheapen the lead up to war and the birth of the Stormcloaks. Few if any of the other lore books have their authenticity questioned, yet because of its proximity to the civil war people find it invalidates the source and instead substitute their own understanding as an alternative.

Ulfric may or may not be innocent of the charge of issuing any orders, but he still presided over the incident in 4E 176 when the retribution took place and held a seat of power, not only as a dominate military figure in the Reach but also as the de facto ruler. I think it's pretty unlikely that he was unaware of what was taking place and that he didn't do anything to stop it is at best suspicious. It was also still his militia that did the deed, men who he commanded the loyalty of and would have likely held little reason to take orders or suggestions from Igmund. I do however see no reason why Igmund would have not had a role in the incident, particularly when he regained power between Winter 4E176 and sometime in 4E177, especially considering his persecutions of the Forsworn past 4E 200. I think it also bares thought as to what Ulfric's motivation could have possibly been to have done such a thing, certainly Igmund would have far more motivation to take such measures as it was his city and father who had been killed and occupied, where as Ulfric was just the Son of a Jarl who got impatient with the situation and charged in without any real stake in the situation.

You do bring up a good source though, the Forsworn themselves. Who are the people who say such so I can look into it?

I think the Void Nights also deserve a mention, because the Khajiit attribute the return of the moons (which they depend on as a part of the lunar lattice) as the doing of the Thalmor and the Aldmeri Dominion, which is why the two city-states that Elsweyr broke into after the assassination of the Mane (the Khajiiti rulers) aren't really part of the empire anymore.

In truth I couldn't find much on the void nights, although I did give it a mention. If you have any references that go further than I did i'd love to expand upon it, especially given that it was such an important event to the weakening of the Empire and strengthening of the Dominion.