Board Thread:Lore Discussion/@comment-9062114-20150428153757/@comment-26357188-20150810103448

Well, Serana did only partially soul trap the DB, but I agree that it is pretty illogical. In fact, when you meet Durneheviir in the Soul Cairn, he tells you that not even the Ideal Masters who are Daedra lords were able to extract his soul. If Serana actually could soul-trap the Dragonborn from a lore perspective, she would need some sort of Aedric power, since Daedric power is not enough. And I highly doubt some vampire has the power of the divines.

But then again, the whole of the lore in Dawnguard was treated very poorly compared to the rest of the game.