Quest boss

From Diablo Wiki

Quest bosses are superuniques (and sometimes act bosses) who must be defeated to finish quests and advance through the game. It can be any significance of a boss, such as the Butcher from Diablo I or Diablo himself. Only one is known so far (King Leoric), but more will be added as they are revealed by the Diablo 3 development team.

Leoric

The Skeleton King returns in Diablo III, and he's up to his old skeleton-summoning tricks. He's found sitting on his throne, must be awakened by the use of a quest item, and does not return in a good mood. Leoric wields a gigantic hammer and commands a vast army of the skeletal undead. He is a quest boss and was the final creature to defeat in the BlizzCon 2008 demo build. See The Skeleton King quest article for more details.

Diablo III Monsters [e]
Animals: Animals Beast Desert Dervish (S) Dune Thresher Goatman Goatman Shaman Goatman Spearthro. Lacuni Lacuni Huntress (S) Lacuni Warrior (S) Lamprey Sand Wasp (S) Scavenger	Demons: Demons Activated Vessel Berserker Cultist Dark Demon Dark Vessel Fallen Fallen Imp (J) Fallen Shaman (J) Fallen Lunatic (J) Fallen Overseer (J) Fallen Hound (J) Frozen Mage (S) Gnarled Walker Malformed Poison Mage (S) Tomb Viper (S)	Undead: Undead Crawling Torso Bone Reaver (J) Ghoul Grotesque Skeleton Skeletal Archer Skeletal Executioner Skeletal Shieldman Skeletal Summoner Unburied Walking Corpse Wraith Wretched Dead Zombie	Bosses: Bosses Baal Diablo Mephisto King Leoric Thousand Pounder Siegebr. Assault Beast Mistress of Pain Husam (A)	Objects: Objects Destructibles Ghostly Orb More Monster Info Monsters Basics Monster modifiers Quest bosses Superuniques Fan-made monsters
(All categorization is tentative, pending more comprehensive game information.) (J) = New in June (S) = New in September