Tathamet

Tathamet is a malevolent Titan from Diablo lore, the "Prime Evil" itself and the source of all darkness within its setting - originally a seven-headed dragon second only to Anu ("God") in power it was slain and from its seven heads were born the Great Evils that would come to rule Hell and plague humanity and the Higher Heavens.

Tathamet was slain long before the mortal world came to be, as such he is never encountered in the game - however in Diablo III the titular Diablo seeks to become the new "Prime Evil" and follow in Tathamet's footsteps.