Nihlathak was the last surviving Elder of Harrogath. He starts out as an NPC in Harrogath, but after the player completes a certain quest, Nihlathak becomes an enemy. Nihlathak struck a deal with Baal, the Lord of Destruction. His Corpse Explosion and Arctic Blast skills are the reasons he is feared among Hardcore players. Once defeated, a crevice opens beneath his skeleton and drags him inside, implying that he was dragged down to the Burning Hells.
Diablo Villains

The 7 Great Evils
Duriel | Andariel | Belial | Azmodan | Mephisto | Baal | Diablo
Other villains
Archbishop Lazarus | The Butcher | Skeleton King | Warlord of Blood | Na-Krul | The Dark Wanderer | Blood Raven | The Summoner | Izual | Nihlathak | Maghda | Ghom | Rakanoth | Cydaea | Adria | Zoltun Kulle | Tathamet | Malthael
Great Conflict | Sin War