Before you knock this idea, saying it's impossible, I would like you to hear me out. Mankar Camaron was a High-Elf who founded the Daedra worshiping cult called the Mythic Dawn. This cult would be responsible for the death of Uriel Septim VII and the end of the entire Septim Dynasty, even if the end of the Septim Dynasty wasn't quite direct to them. He appears in, The Elder Scrolls IV: Oblivion. (This sentence is a potential spoiler for, The Elder Scrolls IV: Oblivion) You may not have ever beat this game, but if you played the main quest at least a little bit, you may remember him as the leader of the Mythic Dawn who escaped through the portal when you tried to infiltrate the Mythic Dawn after getting Martin Septim to the blades. Something you may remember is that Mankar Camaron had the unusual ability to wear the amulet of kings. For those of you who don't know, the Amulet of Kings is a gift from the Aedra Akatosh, who bestowed it upon humanity to the empress of the Imperial Empire, not the Nordic Empire, or the Elf's Empire before the Imperial's Empire. It was somewhat of a covenant between the Aedra, the Divines, and Mundus, the realm of Mortals, containing Nirn. It was supposed to protect Nirn from the Daedra, so long as the heir of people such as Tiber Septim still wear it and the Septim bloodline still rules the empire. Mankar Camaron can wear the Amulet of Kings, though he is not a Septim, that is not what I am implying, in fact, his mother and father are known. Mankar Camaron simply has the ability to wear the Amulet of Kings, strange as it may be. If you still need proof, if Mankar Camaron had been a Septim, when he took the Amulet, Merunes Dagon would not have invaded Nirn, the Septims would still be in rule of the Empire and a Septim would still have the Amulet. Those were the terms of the Covenant after-all. What strangely gave him the ability to wear the Amulet was that he was Dragonborn. In the Mysterium Xarxes, the books he wrote, he claims being able to speak a new language and soon being able to speak fire. As you may know, dragons in The Elder Scrolls speak a language, when they say these words, they have a powerful effect, as a result a dragon's fire breath is the result of a word or words of power. So, if what he describes is in-fact the thu'um, then he is dragonborn, seeing as there was no-one else to teach him and he appears to have learned to do this rather quick. Like I said, he did learn it fast and only Dragonborn can learn the thu'um so easily, other Dragonborns are even mentioned in TES Skyim, there is a line implying that it is unknown if you, the player, are the only Dragonborn, as there may be more in other places. (sorry, I can't dig up this one line, I found it on my first play-through of Skyrim.) To conclude this rather long paragraph, if there were only one lines of Dragon blood in humans, that being the Septims, then you couldn't have been Dragonborn in Skyrim, so who did your Dragon blood come from. As we have learned form the Septims, the Dragon blood, is indeed passed down through generations of blood-related-family. Your race is optional in Skyrim, so the race you chose cannot deny this theory, even so, he could have had a descendant who bred with a person of different race, resulting in your character (Aside from beast races, Argonion and Khajiit). Since (Potential spoiler for The Elder Scrolls IV: Oblivion) you witness the death of the final Septim, Martin Septim, at the end of TES Oblivion, you cannot be a Septim in Skyrim. The only bloodline left to make a Dragonborn would be Mankar Camaron's bloodline. If you were a Septim, none would be able to deny you that, and you would still be in power, instead, you are not a Septim, you are of descent of Mankar Camaron. (Theory)