The Diablo games take place in a dark fantasy world known as Sanctuary, the mortal realm which lies between Heaven and Hell. In this world the player fights hordes of demons, undead, and other monsters to slay the evil which is corrupting the lands.
Diablo - Lord of Terror
Diablo is one of the three Prime Evils of the Burning Hells. As the Lord of Terror, he is responsible for all of humanities fears, and filling humans with bloodlust. Although Diablo is younger than his two brothers, he is the strongest, and essentially the de facto leader of the three.
Baal - Lord of Destruction
Baal, the Lord of Destruction, is one of the Prime Evils who rule over Hell in the Diablo series. Despite being imprisoned within the body of the mage Tal Rasha, he eventually laid siege to Mt Arreat in the hope of destroying Sanctuary's most guarded secret: The Worldstone.
Mephisto - Lord of Hatred
Mephisto, the Lord of Hatred, is one of the greater evils that the player must battle in Diablo II. He is the eldest of his brothers but was the first to be sealed inside a soulstone by the Horadrim. Once Mephisto broke free from his shackles he set about corrupting the jungles of Kurast and forcing the high priests of Kurast, the Zakarum, into doing his bidding. He took the strongest of the priests, Sankekur and took over his body, essentially resurrecting himself in physical form. The Zakarum proved useful to Mephisto as they helped spread his evil influence throughout the area.
Andariel - Maiden of Anguish
Final boss of Act I in Diablo II.
Duriel - Prince of Pain
Final boss of Act II in Diablo II.
Azmodan - Lord of Sin
Final boss of Act III in Diablo III.
Belial - Lord of Lies
Final Boss of Act II in Diablo III.
Deckard Cain - Last member of the Horadrim
Tyrael - An Archangel
The Butcher is (usually, though not always) the first boss in Diablo, who resides in a small corpse-filled room and wields a large cleaver which he uses to maim and kill his victims. The Butcher utters his signature line, "Ah, fresh meat!" and attacks, wielding his namesake weapon: the Butcher's Cleaver.
In Diablo III, it is revealed that several "Butchers" exist. A new Butcher is the final boss in Act I of Diablo III.