Curiosity got the better of me. Took a little bit of digging, but here's a story about a guild called Conquest and their adventures in Sleeper's Tomb:
Congrats Conquest
Thread #40 in the newspost explains what happened: Conquest was allegedly using an exploit to kill a warder, and SoE decided to unleash the sleeper on them. Sleeper didn't die.
I bet a lot of myths came from this.