As others have said, it was the WotLK Pre-Launch event, which coincidentally overlapped with Halloween, I believe.
There actually was a quest to kill him involved, but the quest wasn't what spawned him as far as I'm aware. He was just there; and the quest was a separate beast. One of my alts still has it completed in her log, and has been unable to turn it in for years (at least not through any means I'm aware of). I killed him with her and forgot to turn it in before the event ended, as it was given by an event NPC, she never finished it. I believe it's called "Chamber of Secrets" if you want to look into it.
You have me insanely curious to take that alt in there and see if the door opens though now. I don't expect it to... but I'm still going to try when I get home, lol. If it works, I'm poised to make a zillion gold passing out Vampiric Batlings to people (but if that were possible, I'm sure the entire community of this site would be perfectly aware of it by now, so again, I don't expect it to work).
I will forever regret passing on the guitar axe to another hunter in my guild. He used the "it's an actual upgrade for me" excuse (we used axes back then, kinda)... in hindsight, it was an upgrade for about 10 minutes before WotLK hit, and much more coveted as a toy than an actual axe in the longrun. I don't even know where that guy is now, or what his name was, he probably quit playing. I should not have been the nice guy that day.