Yeah, and I had it coming something gonna be mentioned about Serana. Yes, I know she's the daughter of 2 vampires, it's stated she was once human, then when Molag Bal offerred his gift, this made her, and her entire family pure-blooded vampires.
You guys must referring to Dengeir of Stuhn, in Falkreath. His long dead ancestor Vigharjust turned to a vampire which is why he gives you a quest to slay his ancestor. Probably he turned which is why he's still alive. Hope it rings a bell :)