User blog comment:Zippertrain85/Dawnguard Conclusion/@comment-17114085-20140511164121

He would need to have been related to Ysgramor

Not necessarily since before Harald there was no High King so Skyrim was not unified. There were other Kings ruling the rest of Skyrim. Beside only the Kings from Windhelm and Winterhold were related to Ysgramor, the others weren't.

He could also have been a King after the Ysgramor Dynasty. There are many blanks in the line of High Kings after Olaf One-Eye. Not to mention that after the Ysgramor Dynasty parts of Skyrim got conquered by the other provinces. Falkreath for example because part of Cyrodiil, and had a King.

The "sacrificing thousands" part is highly unlikely, a genocide that large would have been recorded in history. So that part at the very least was exaggeration on Harkon's part. Besides Serana and Valerica said that they became pure-blood vampires because they were cultists of Molag Bal and gave themselves to him.