Agahnim
When Hyrule became besieged by an onslaught of terrible misfortunes (pestilence, etc), the king offered a reward to anyone who could end their suffering. A mysterious stranger named Agahnim came and rescued Hyrule with his magic. He was declared a hero and given the pretigious positions of chief advisor to the king and heir to the seven wise men. Agahnim used these positions and his magic to brainwash all of Hyrule's soldiers, dispose of the King of Hyrule and make the decendents of the seven wise men disappear to the Dark world (where Ganondorf is imprisoned).
After being saved by Link, Zelda hides in the sanctuary until Agahnim's troops find her and take her to the tower at the top of Hyrule Castle, where Agahnim sends here to the Dark World, thus breaking the seal the seven wise men put on the Dark World. Before Link can soundly defeat Agahnim, Agahnim draws him into the Dark world. As Link rescues the seven maidens in the Dark World and in his travels, learns that Agahnim is a servant of Ganondorf.
When Link defeats Agahnim at the top of Ganon's Tower in the Dark World, Agahnim collapses and from his body a faint image of Ganon appears. The image turns into a bat and flees to the Pyramid of Power.
Agahnim's relationship with Ganondorf is occasionally debated. Some people believe that Agahnim was just a wizard who Ganondorf corrupted and used to do his work in the Light World (Hyrule). Others maintain that Agahnim was merely a disguise used by Ganon. This theory is supported by the fact that Ganon appears from within Agahnim's body and that in the final battle Ganon refers to Agahnim as his alter-ego.
When attacking he throws his magic attacks against Link. The first of them, a magic ball, can be hit with the sword to reflect it, so that it hits Agahnim himself and wounds him. The second one can also be hit with the sword, but then it splits up and flies through the room. The third one is a lightning which Link can only avoid by standing at Agahnim's side. While he is alone in Hyrule Castle Tower, he creates clones of himself to irritate Link.
Agahnim also made a small cameo appearance in Link's Awakening, appearing as one of the forms of the final Nightmare in the Wind Fish Egg. Here he can be defeated the same way as in the previous game.