So, after months on this site of debating Dawnguard lore (which frequently resulted in arguments) I have decided to make yet another blog on it, addressing some of the criticism and giving a conclusion to my beliefs in the field. So, let's begin.
Starting with Serana, no I do not believe she is from the First Era. As there is no solid proof for this in my opinion, yes there's some speculation that possibly shows that she might be from that time period, but nothing to show that she really is. Even with evidence contradicting it, since he age is measured in centuries, when it should've been measured in milleniums, if she was actually from that time. I believe she was probably born at the time of ESO since Molag Bal was a big influence at the time, and the Empire did not exist (Yes, the point of that time period was that there was no EMPIRE in Cyrodiil, though there was still a government), it would also explain why her age is measured in centuries, since it wasn't quite a millenium from thre current date in The Elder Scrolls V: Skyrim. An argument against this I hear is that Durnehviir knew Valerica when she first came in, therefor she's from the First Era, this though isn't that solid since it is possible Durnehviir could've been from before that. Some Dragons did live after that time period, and it would also explain why he needed strength, since he was living in a time where his comrades are almost completely gone and people and looking to wipe him off of the planet.
The next is Harkon, I do not believe he was a king of anything really or that he "sacrified a thousand innocents to Molag Bal", both of these stories completely contradict lore. If Harkon was a king and from the First Era, he would need to have been related to Ysgramor, but this is never referenced, once again another argument to show they are not from the First Era. But back to point, if he's from the ESO time period there would've been some piece of history about him, but there's none, no reference to him ever being a king in Skyrim. He couldn't have been a Jarl, since those are not really kings, but rather lesser lords who serve the king. Also, how could he have gotten away with sacrificing a thousand people? There would've been a rebellion or something if that happened, people don't respond well to that stuff. It also would've made him notable as a king, if he was responsible for committing that atrocity, yet once again he nowhere to be referenced.
To be honest, this was most likely due to Harkon's large ego, he most likely lied about these stories to gain some sort of respect outside of his Vampire Persona, Lord Harkon. But for the reasons I explained the stories were poorly thought out and quite frankly I don't consider canon at all in the universe of Elder Scrolls.
But, by all means, leaves your comments down below, if you want to agree with me, refute my arguments, call me stupid, make fun of me, or whatever you would like. :)