In the original Diablo, the game’s final boss is Diablo.
Diablo is the Lord of Terror and the central antagonist of Blizzard North’s 1996 action role-playing game. The player descends through the cathedral beneath Tristram, fighting monsters and discovering the history of the fallen king Leoric and the demonic corruption beneath the town.
At the lowest level, the player confronts Diablo himself. After his defeat, the game’s ending depicts the hero placing the Soulstone into their own forehead, an act that suggests Diablo’s influence has not truly ended. This ending helped establish the darker, cyclical storytelling associated with the series.
Baal and Mephisto are Diablo’s brothers and fellow members of the Prime Evils, but they are not the final boss of the first game. Andariel appears later in Diablo II, not in the original Diablo.