Somebody needs to replay WC3.
Mannoroth was the pit lord who originally corrupted the orcs. In WC3 he corrupts Grom Hellscream's clan, causing them to kill Cenarius. Thrall saves Grom and the two of them fight Mannoroth, with Grom getting the killing blow.
In The Frozen Throne, Illidan and company finds Magtheridon at the Black Temple. Illidan defeats the pit lord and claims the temple for himself. It was assumed Magtheridon was killed until Burning Crusade came out.
Mannoroth was the pit lord who originally corrupted the orcs. In WC3 he corrupts Grom Hellscream's clan, causing them to kill Cenarius. Thrall saves Grom and the two of them fight Mannoroth, with Grom getting the killing blow.
In The Frozen Throne, Illidan and company finds Magtheridon at the Black Temple. Illidan defeats the pit lord and claims the temple for himself. It was assumed Magtheridon was killed until Burning Crusade came out.