User blog comment:Poisoned apples/The Dark Heart of Skyrim, the Next Story in ESO/@comment-1470368-20200202173956

Can't wait 'til it's revealed that this is the origin story of the Volkihar Clan. It seems likely to be the case, and I'm sure that they'll find some explanation for Castle Volkihar being located far north of Haafingar Hold by TES5's time. Especially since Harkon, through pledging himself to Molag Bal, is the progenitor of the clan.

There are historical contradictions to the story Harkon tells about his time before vampirism, but it's easily inferred that the "age long forgotten to history" is none other than the Interregnum [the "Days & Nights of Blood & Venom"] time-period that TESO is meant to be set in.

Besides, there's no doubt that Serana wouldn't be familiar with any "empire" in Cyrodiil if she was born during this long period; as what's left of the actual Second Empire in the 2E580's is just Cyrodiil [formerly ruled by Clivia Tharn].