The Butcher:
The Butcher is a cleaver-wielding murderous demon, and one of the scariest found in all of Diablo. He occurs in every multiplayer game and half of single player games, and is located within the butcher's block on level 2.
The Skeleton King:
The undead body of King Leoric must be defeated to complete his quest and free his followers from his malign influence. He is huge and dangerous, capable of raising skeletons to aid him in the battle. This quest is present in every multiplayer game somewhere on level 3. In singleplayer this quest occurs half the time, and when it does the Skeleton King is found on a sub-level, from level 3.
Gharbad The Weak:
This talking, Goatman is found on level three, but only in single player games. Gharbad isn't very difficult to defeat, but his quest is one of the most memorable in Diablo thanks to his amusing dialogue and pugnacious nature.
Zhar The Mad:
This singleplayer-only quest takes place on level 8 in the Catacombs, when a player finds Zhar the Mad in one of the library rooms. Zhar must be destroyed to complete the quest. Zhar always drops a spellbook when defeated.
The Warlord of blood:
The Warlord of Blood is a single-player only Steel Lord boss found on level 13. He is the most valuable quest boss in the game, not for his item, but for the armory of (random magical) 4 weapons and 2 plate armors he guards.
Lachdanan:
Lachdanan is an NPC/monster found on level 14. Lachdanan is a Steel Lord that doesn't fight, and instead asks the player to find a golden elixir that will free his soul from the damnation placed on him by the Mad King Leoric.
Archbishop Lazarus:
Archbishop Lazarus is the betraying priest who did much to bring about the fall of King Leoric and the destruction of Tristram. Lazarus is found on level 15 in multiplayer, or in a special sub level reached from level 15 in single player. He must be killed to complete the Lazarus quest and to advance down to level 16.
Red Vex:
Red Vex is found beside Lazarus in single-player and multi-player games. She doesn't have to be killed to complete the quest, but she has nice loot, so it's well worth it.
Black Jade:
Black Jade is found beside Lazarus in single-player and multi-player games. She doesn't have to be killed to complete the quest,, but she has nice loot, so it's well worth it.
Diablo:
Diablo is the final boss, and must be defeated to end the game. He's found inside the eastern quarter of level 16, and once he's dead the game ends. He is susposed to be the toughest. He is well guarded my his many minions, so be careful.
