A Horde raid faces Roman'khan (prior to patch 2.3)
A Silithus outdoor mob which is spawned by a quest. Prior to patch 2.3, he was much stronger and required a small raid to kill.
He doesn't drop anything but the item required for the quest.
Emissary Roman'khan is spawned by the quest [60G] The Calling that requires you to kill him and loot the [Crystal Unlocking Mechanism]. This encounter was designed for 20 people, but the quest would allow you to do it with up to 40.
Abilities
Wilt - An AOE ability that locks on to several mana-using raid members (around 10) then hits them repeatedly, dealing 500-ish Nature damage (resistable) and draining mana constantly (not resistable). Every point of mana drained will restore a large amount of mana to the Emissary. It is recommended to keep HoTs on the people targeted by Wilt.
System Shock - Single target damage spell that will blast a random person for 1500-4000 damage, and heals Roman'khan for a small amount. As Roman'khan gains mana, he uses this ability more often and it does more damage.
Explode - When Roman'khan's mana bar is full he will use this ability for massive AOE damage. For this reason, it is recommended to keep mana draining Roman'khan, unless you have a large raid that can dps him down.
Drain Life - Whenever a player dies due to Roman'khan's abilities, he will regenerate 2% health.