Baal is the final act boss of Diablo II.
He is the Lord of Destruction and the youngest of the three Prime Evils. In the fifth act, players pursue Baal through the snowy region of Mount Arreat and eventually confront him in the Worldstone Chamber.
The fifth act was added by the Lord of Destruction expansion, which also introduced the Assassin and Druid classes, two additional character class types, and new equipment systems. Baal's defeat concludes the expansion's main campaign, although the game continues through difficulty levels and replayable character development.
Diablo and Mephisto are also Prime Evils, but they are bosses in earlier acts. Diablo is fought at the end of Act IV, while Mephisto is defeated at the end of Act III, so neither is the final act's boss.