This is something that's always bugged me about Oblivion. To open a portal to Paradise, you need four things: The blood of a daedra, the blood of an aedra, a Great Welkynd Stone, and a Great Sigil Stone.

The Champion of Cyrodiil is able to obtain three of those four items with no complaint from me. I personally find it rather hard to believe that there's only one great weklynd stone left in Cyrodiil. Presumably, there were many of them created by the Ayleids, but they were all plundered except for one. But even if that were the case, the stones should still be out there somewhere, in the hands of private collectors or such. So how is there only one stone in the entire game that can be used?

But at least I'm not left questioning how Cameron made his own portal.

But then we have the blood of an Aedra. The only way the Champion of Cyrodiil was able to obtain any such blood was by entering Sancre Tor - whose original defenses were still left unscathed despite centuries of every other dungeon in all Tamriel having been plundered a thousand times over - and retrieving the armor of Tiber Septim, who became a divine after he died.

The game makes it exceptionally clear that this is, literally, the only sample of Aedra blood in all of Nirn. You can hand over any of the daedric artifacts you want in order to fulfill the "Blood of the Daedra" criterion. But the Aedra's blood? It's Talos' armor or nothing.

But if that were the case, how did Cameron make his own portal to Paradise? You know ... when you first infiltrate the Mythic Dawn hideout? Where did Cameron get his Aedra blood from?

There's only one way I can think of to rationalize how Cameron was able to create this portal: The Amulet of Kings. The amulet is said to be imbued with the Blood of Akatosh, which explains why the soul gem in the center is red instead of silver.

So at least with Mankar Cameron himself, it makes sense.

The only problem is ... he's not the only inhabitant of Paradise. He isn't even the only mortal inhabitant. We see several mythic dawn agents for whom this is their afterlife. In fact, it's only thanks to one of them that you're able to escape the Forbidden Grotto. This ship-jumper explains that he was at the Battle of Kvatch and realized the error of his ways right before he died.

So how did these people get to Paradise without any aedra blood?

Magic

To begin, Camoran's commentaries mention "four keys." A https://www.imperial-library.info/content/translating-mysterium-xarxes of the Mysterium Xarxes pages that can be glimpsed in Oblivion show that the identities of these keys -- a Great Welkynd Stone, a Great Sigil Stone, the blood of a Daedric Prince, and the blood of an Aedra -- are cryptic in their description:

"In my first arm, a storm. My second, the rush of plagued rain. The third, all the tinder of Anu. The fourth, the very eyes of Padhome."

It's possible that Martin's answers aren't the only "correct" ones. Morian Zenas thought that, with the destruction of the Battlespire, access to the Soul Cairn had been lost, but Valerica had found another way over an era before.

The Mythic Dawn instructs prospective members to find the four volumes of Camoran's Commentaries to locate the Mythic Dawn's shrine to Mehrunes Dagon. In a similar vein, the instructions within the Xarxes itself may be another "test" -- how to enter Paradise (perhaps how to enter an Oblivion realm, if Paradise is just a pocket of the Deadlands) without its owner's permission. Others have traveled to the Princes' realms without being invited, of course, but the inscrutable Daedra may have been more than happy to allow it. An Oblivion gate is akin to a door left ajar -- if the Daedra retreated from the Argonians during the Oblivion Crisis, it goes to show that the Princes don't always abide trespassers.

It's worth noting that Camoran himself did not appear to require the four keys to travel to and from Paradise after the realm's creation. He could not have had any Daedric artifact other than the Ogmha Infinium, at least, because the player cannot acquire that before "Blood of the Daedra" begins.

***

In the original plot of Oblivion, you worked with an agent to purchase a Great Welkynd Stone, rather than plundering it.

Martin states that they're of great value to mages and occultists. It's not necessarily that there are none in Cyrodiil whatsoever but rather that it may be impossible to locate someone who owns such a stone or that the owner may refuse to part with it. Or maybe the stones have been "used up" through whatever purposes the owner acquired it for.

It's never stated that Tiber Septim's armor is the only source of the "blood of a Divine"; the way Martin discusses it suggests that he was stymied trying to figure out practical means of obtaining the blood.

Martin states that the Aedra do not have artifacts -- or at least, not those composed of their essences as the Princes' artifacts are. Would, say, the Brush of Truepaint, said to be made of Dibella's hair, have worked? What about the Crusader's Relics? Something like Auriel's Bow or the Lord's Mail?

Jauffre knew that the armor bore Tiber Septim's blood and would still be in Sancre Tor, so it was probably considered a surer shot than trying to track down other artifacts.

***

As the Golden Saints and Dark Seducers return to their respective Wellsprings upon their deaths, Camoran has bound his followers to his Paradise. It's not so much that they enter Paradise as that they're dragged there.

Are there any references to this plot anywhere?

Mankar Camoran wasn't trying to open a portal from the outside. The realm was his own creation, presumably with direct help from Mehrunes Dagon. He can create portals out to Mundus because he has the cooperation of the Mythic Dawn members on the other side. Martin has no such cooperation, and has to force things unilaterally, which is why it's much harder.

That also explains why the MythicDawn members have less trouble getting in, even if they don't like it when they get there. Mankar can open a portal any time one of them requests, as having both ends working together is the key. It's also why Dagon himself needs the Mythic Dawn for his gates.

Unused topics, mostly.

Uhm it's pretty clearly explained in Oblivion that the "citizens" of Paradise (other than the Dremora, obviously) are those that fell in service of Mankar... I.e. the Mythic Dawn people you've killed previously in the game

Ebony is Lorkhan's blood. You could just use some random locket, mace, boot, or what have you.

Also, it's Oblivion. Thinking about things too much (or even a moderate amount) ruins everything.

That's not quite right. Correct me if I'm wrong, but Lorkhan isn't an Aedra, and ebony is merely said to be his blood.

I'm pretty sure that, minus Mankar, Lorkhan would be considered an Aedra even with his Padomic leanings. A portion of his divine being was instrumental in the creation of Mundus (his Heart), so you could make the argument that just like the other Aedra he gave a piece of himself to make Mundus possible. Daedra by contrast did not give anything to the creation of Mundus. However, being a dead god, I would imagine that his fossilized blood probably wouldn't have the juice needed to open the portal.