I don't think Lord Harkon was entirely honest with your character through the entire vampire questline. He does lie to you the entire time though. Making you a vampire lord is just a ruse to make your character believe that you're part of his coven when he originally plans to kill you anyway, since he thinks he's all powerful. He makes you his errand boy and than sits back while you go get Auriels Bow so he can kill you for it. He never planned on keeping you or Serana around. Kill you, take the bow, kill and sacrifice Serana and cast eternal darkness over Nirn.
Is it just me or is Harkon's backstory beyond ridiculous? It not only completely contradicts Elder Scrolls lore, but just plain doesn't make sense.
I know Harkon is old, however he is from the first era, the empire was not even created yet, skyrim was not unified, there might have been many kings, and as for the sacrifice thing, he could have just offered all of the people he ruled over. And again it was first era, you would be lucky to find any real text from that time period.
Is it stated at any point during the game that he has always lived in Skyrim? I don't think so. He might have been a King somewhere else? Would also explain the "previous owners of the castle" thing, he could have moved there from his bithplace at any time since the first era. But even Serana contradicts her own father.
Serana has made comments that they have been in Skyrim ever since they were turned. Because she was locked away shortly after with the elder scroll.
He claims he was a mighty king, of what? Haafingar? The island his castle is on? You can't just say he was a king but not say what he actually ruled. Also, don't you think people would remember him? He is literally never talked about anywhere. No lore about Skyrim says a thing about any Harkon, definetly not being a king of any sort. As a matter of fact, their are no kings in Skyrim. They have Jarls instead. You could give the argument that he was a Jarl, but why would he call himself a king instead of a Jarl? He claimed his "land was vast, and wealth seemed limitless", a Jarl only controls their hold. They now exactly how much land they have. It can be nice land, but it certainly wouldn't be considered "vast" when you have all of Skyrim to compare it too. Though Jarls are wealthy they probably don't have enough to consider it "limitless" .
Remember, Jarls are lessor governers below the High King, why would one Jarl be given a ginormous amount of wealth? Serana also talks about the previous owners of the castle numerous times. If Harkon was the king of the land I would've assumed he and his family would've lived like royalty their entire lives. You see, their are so many loose ends that it's ridiculous. But wait, their's more.
He also said he "sacraficed thousands in order to get turned by Molag Bal" . First of all, how could he get his hands on a thousand people? You can't just pick up a thousand people off the street. It would take years. And by the time he even thought about turning into a Vampire he was already old. Second of all, you're trying to tell me no one would notice? A thousand people is a lot of people. Enough to level a city (by Skyrim's standards). How would he possibly get away with this? Their would have to be an investigation or something. They probably would've sent the army after someone like this. This could make him the biggest murder in Skyrim's history. Killing 250 people is considered record breaking, just think of one thousand. Their is no way in hell he got away with killing this many people.
How could Bethesda have left all of these loose ends? Did they just get lazy or something? Maybe we should at least leave open the possibilty Harkon might have been lying about his past. Because I find it hard to believe with that he was completely telling the truth, despite all of these flaws.