"Hello, Guardian. I must say, it really is great being a Vampire. You get to live forever, you don't have to eat or breathe, and you can fall from any height. But there is one Vampire who is a stain on all of our kind. When CluDathoon attacked, the ancient vampire Demona went on a rampage. Now, she has taken up refuge in what remains of Valdrec's floating island. This land need to be rid of her, once and for all. Travel to the ruins of Valdrec's floating island, and kill Demona."
Exit the Resistance HQ and head to the left. Let your compass guide you and continue walking until you reach the remains of Valdrec's island, no long floating. Demona is located inside the building. She and her vexes will attack you as approach the entrance. Once you have killed Demona, return to Resistance HQ. Speak with Nish for the next quest.