Serana needed help out of Dimhollow Crypt and back to her manor, and I was happy to oblige. That's just what wandering adventurers do. After being stuck in the cave for longer than I care to admit (thanks for pointing me in the precise wrong direction, Clairvoyance spell) I battled my way out. Though, to be perfectly honest, I didn't have to do much of the battling. A bodyguard assassin, a vampire princess, and her conjured warriors tend to make a pretty potent mixture of butt-kickery.
I guided Serana to the opulent Volkihar Keep--how had I not noticed
Command of the undead, you say? That's tempting