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
Andariel | Azmodan | Baal | Belial | Diablo | Duriel | Mephisto

Other villains
Adria | Archbishop Lazarus | Blood Raven | Cydaea | Ghom | Izual | Maghda | Malthael | Na-Krul | Nihlathak | Skeleton King | Tathamet | The Butcher | The Dark Wanderer | The Summoner | Warlord of Blood | Zoltun Kulle