http://elderscrolls.wikia.com/wiki/Septim_Dynasty
The septim dynasty refers to the bloodline of Emporers and Empresses beginning with http://elderscrolls.wikia.com/wiki/Tiber_Septim. Also known as the "Third Empire", this empire spanned the entire http://elderscrolls.wikia.com/wiki/Third_Era. The majority of these rulers are considered to be Imperials, unless otherwise stated.
http://elderscrolls.wikia.com/wiki/Amulet_of_Kings
Nothing mentions a Dunmer empress.
Read history of the Empire, Katariah. Dunmer Empress. You can also read the Armorers challenge, it's in there.
A few more things:
- None of the Septims were direct descendants of Tiber Septim, this means being Dragonborn is not necessarily due to birth.
- Alessia and Tiber are not related in anyway
- The Reman emperors were not related to Alessia or Tiber in anyway
- All three lines lit the Dragon Fires
- The dragon fires are no longer relevant, the amulet is destroyed. Akatosh replaced the function
- It is entirely possibly that Mankar was dragonborn as well, he is the descendant of an ancient royal line in Valenwood.
- Being Dragonborn was never important, it was being able to light the Dragonfires. Naturally a dragonborn could do that, but the point was that if you wore the Amulet of Kings you were blessed by Akatosh, if you could light the Dragon Fires you were blessed by Akatosh. This blessing was needed to save the realm from a Daedric invasion, that possibility has been stopped. So being Dragonborn is no longer relevant.
- The only way your character would have greater claim then somebody else is if either you are a Septim or if you conquer the Empire. Other options no matter how cool are irrelevant.
What everyone needs to remember is that there are two types of Dragonborn. There is the true Dragonborn, one who has the soul of a dragon. This would be people like Reman Cyrodiil, Tiber Septim, and the player character of Skyrim. Also might include St. Alessia, Uriel VII, and Martin. They can absorb dragon souls and learn shouts easily. The other type is dragonborn by blood, aka one who has the blood of the dragons. This is more figurative. It is a mark of legitimate authority and a sign of Akatosh's favor to rule. They do not, however, have the soul of a dragon. None of this seems to be passed by blood otherwise none of the Septim emperors after Pelagius I would have had the dragonblood and been able to light the Dragonfires.
The Amulet of Kings and the Dragonfires only needed someone of dragonblood, aka endorsed by the Divines. All true Emperors are of the dragonblood but very few are actually Dragonborn and all Dragonborn are of the dragonblood.
Mankar could have been either of the two.
Simply not true, being dragonborn or dragonblood nothing matters it's all the same. How or why the blessing is given is the question. Akatosh can give it to whom he wants when he wants, it is also possible that Akatosh gives it to the people that need it, like the people that are about to light it. Or the people that really want it, this would explain people like Katariah, Tiber Septim, all other Septims and Alessia