Use the Sharp Wooden Stake to kill Blood-Thane Lucard in Dreyrgrot.
Most Fascinating! It would appear that we're dealing with our very own vrykul vampyr! All we have encountered have been of blood elven descent, but it has long been theorized that the vrykul have suffered the curse as well. It's too bad he's awake - I would dearly like a subject to study. Oh well, guess there's nothing for it. Take this stake and put it through the old chap's heart. I'm pretty sure that should do the trick.