Castlevania is a game where you have to go in a scary castle and fight Dracula. My husband owns it, and, apparently, this boss seriously harkens back to that game.
Prince Tenris Mirkblood is available in Karazhan (a level 70 10-man) after you kill Attumen the Huntsman. Just keep going past Attumen's body, clear the mobs around the repair guy, then take a left up the stairs and clear the circle so you can kite him (kite = keep him running, following you). There's a door you open to unleash the boss. When he's up, watch his portrait and stop attacking when he changes form, because it will kill the person in your group that he changed into.
When he's dead, everyone gets a little bat pet. It's not particularly cute, but it is only available for another week, and then you'll never see it again. So it'll probably be a Feat of Strength.
Make sure to get the quest for him in Eastern Plaguelands, Light's Hope Chapel. It's completely sharable.