Each of the four acts in Diablo II, and the fifth act added in Diablo II: Lord of Destruction, ends with a boss battle. These bosses must be defeated in sequence to advance to the next segment of the game. Act Bosses are the most challenging single monsters in the game (aside from the patch-added Uber Monsters and certain immunities/abilities for bosses against certain characters). They also have better chances of dropping uniques/set items than any common monster.
Click the linked names to see a page devoted to each boss, with info about their stats, skills, and abilities.
The act bosses are
Andariel, Maiden of Anguish, boss of Act 1.
Duriel, Prince of Pain, boss of Act 2.
Mephisto, Lord of Hatred, boss of Act 3.
Diablo, Lord of Terror, boss of Act 4.
Baal, Lord of Destruction, boss of Act 5 and 'guardian' of the next difficulty.
