One of seven, yes. The Prime Evils are he strongest. Mephisto, Lord of Hatred, Baal, Lord of Destruction, and Diablo, Lord of Terror. They reigned over the Burning Hells until the Lesser Evils united and overthrew them, beginning the Dark Exile... though Uncle Deckard had a theory that the exile itself was one of Mephisto's machinations in an attempt to corrupt humanity to the Hells' side of the Eternal Conflict.