Agahnim is a dark wizard from the SNES game The Legend of Zelda: A Link to the Past and despite being a pawn/alter ego of Ganon he's pretty much the face of evil in the game. He served the role of antagonist quite well.
The scene where he takes over Hyrule castle (thus the land itself later) and kidnaps Princess Zelda was pretty epic. You felt pretty helpless.
Thankfully you get a pretty epic boss battle near the end of the game to finish off his evil self. And you get to reflect his own dark magic against him which is pretty sweet. One of my favorite Zelda moments.