The Sleeper (aka Kerafyrm) was a one-time per server event from the Velious expansion.
Put simply, you had "The Sleeper" and four warder dragons guarding him. As long as one warder was up, you could kill the others for nice loot indefinately. However, once all four warders were dead at once, the Sleeper wakes up, goes through a buggy-*** script and then.. well, gets stuck somewhere and spazzes until a GM mercy kills him.
The upshot is that the warders won't respawn anymore so the nice warder loot is gone for all time. Other mobs do spawn, but they drop different stuff that isn't quite as sought after. So you had guilds coming to agreements that no one would kill the last standing warder and all would benefit until some other buncha goobers comes along and decides it's their divinely appointed right to awaken him then everyone else gets pissed and you get to read their drama for a week or so. Great fun for all.
The Sleeper was considered
the unkillable mob until fairly recently. On one of (if not the) his last sleeping servers, Rallos Zek, they got a whoooolllleee buncha people together and woke the Sleeper and (after an aborted attempt stopped by a GM) killed him using chain pet tactics. He didn't drop anything or whatnot.. it was just a "see if we can do it" type event. Kudos to them.
In game lore terms, Kerafyrm was the offspring of a red and a white dragon. Although not explicitly named, the EQ RPG books hint that the red was Dozekar who is now imprisoned in the Temple of Veeshan. Kerafyrm was insanely power as well as just insane and it took a war to bring him down in which lots of dragons died. Veeshan laid down the big two laws after that, that (a) one dragon was not to kill another and (b) no dragon inter-racial breeding was to happen. Years later, Lord Nagafen and Lady Vox were discovered to be attempting some illicit nookie and were captured by their peers. Unable to simply kill them due to Law A, they were imprisoned on opposite ends of Antonica where they reside now until some newbie pick-up raid finally kills them.