So by now, you've probably heard of Tenris Mirkblood, the Blood Elven vampire boss in Karazhan who is surprisingly generous with his loot, dropping a Vampire Batling for everyone in the raid as well as a very sweet axe by both the musical and weaponry related terms -- Although only one of those, unfortunately.
Tigole answered a few questions about this boss in a recent post on the forums. Tenris will only be around for about 10 days, which is why he's so generous with the bat pets, and the axe is the only other item on his loot table, if you don't count the Badges of Justice.
So what this means, first of all, is that if you haven't managed to get a group to go beat him down, you'll want to do it soon or lose your chance at a pretty cool pet and a pretty cool -- if rather weak -- novelty trinket. In addition, if you weren't lucky enough to win the roll on the Arcanite Ripper on your first try, it looks like you'll only have one more chance at it. Sacrifices to the gods of dice and/or the Gods of Metal, whichever you subscribe to, are recommended.