Baal

Baal is one of the three brothers who make up the Prime Evils, along with Mephisto and Diablo. He is a major figure in the world mythology, and features in several of the Diablo Novels. It's not yet known if Baal will make an appearance in Diablo III.

In Diablo I

Baal was not seen in Diablo I, but he was mentioned in the world lore, both in the game itself and in the lore-filled manual.

In Diablo II

Baal featured prominently in Diablo II, appearing in the cinematics and plot, though he was not found as a playable monster in that game.

Baal was the Act Boss of Act Five, added in the Diablo II expansion, Lord of Destruction. The story of that game is all about Baal's efforts to reach and corrupt the Worldstone. Though Baal is eventually defeated by the player's hero, his efforts to corrupt the Worldstone were successful, forcing Tyrael to destroy the worldstone to prevent its power being used by the forces of hell. (So says Tyrael, at least. Many players are suspicious of his actions and think he might have had his own, unrevealed agenda. The plot of Diablo III should shed more light on this issue.)

Stats and information:

• Baal, the boss of Act Five.
• Uber Baal, a more powerful version of the demon added as part of the Pandemonium Event, a special realm-only quest added in v1.11.