Renataki, of the Thousand Blades.
Pain is his lifeblood. Fear, his ally. May he one day return and bring joyous bloodshed with him.
Renataki is a rogue, one of several bosses which can spawn during the Edge of Madness event within Zul'Gurub. His special ability is to use vanish and he will randomly appear and ambush someone which will do quite a bit of damage. He is not that hard of a boss, just throw a few rogues on him and have them tank him and you can DPS him down. His vanish will not break on dots.
It can break now and then on AoE damage (also by Warrior's Casting Demoralizing Shout) and bring him out of stealth, so it might be a good idea to huddle up the caster group with an OT and have the mages and other aoe capable stand at the edges to try and make him visible again before an ambush from him, which usually kills a cloth wearer, and then let the OT pick up the aggro and bring it back to the melee group. Before he vanishes, he usually gouges everyone in melee range, so the mages have a bit of time to get into position.
One useful tactic is to sick hunters and/or locks pets on him. They will continue to follow him even after he vanishes, giving you a steady indication of his position.
Renataki drops one of two blue items in addition to his green quest item: