Zedar was a disciple of Torak.
Formerly known as Belzedar, he was a disciple of Aldur until he attempted to stealthily obtain the Orb of Aldur from Torak by attempting to trick Torak into believing he wished to become his disciple. However, Torak's methods of accepting disciples were wildly different from Aldur's. Zedar, along with Ctuchik and Urvon, was a slave to Torak's will. After becoming Torak's disciple, Zedar became known as "Zedar the Apostate". Over the course of the Belgariad books, Zedar attempted to impede Belgarath's attempts to recover the Orb but was defeated. Prior to that, he was ultimately responsible for the death of King Gorek of Riva. Zedar was also the one who found the child Errand through which he succeeded in stealing the Orb from The Citadel at Riva.
Zedar was finally confronted by Belgarath in Cthol Mishrak just after Zedar had killed Durnik. Belgarath attacked Zedar with his fists out of sheer rage, before tearing open the ground beneath them, plunging both men into the very bowels of the earth. Belgarath emerged victorious, having sealed Zedar alive and trapped within the rock. He then prevented his daughter Polgara from trying to kill the traitorous sorcerer.
In his autobiography, Belgarath postulated that he must have been an Arend because his features appeared to be those of an Arend or a Tolnedran, but "no Tolnedran could possibly mangle the language the way he did". Like all of Aldur's disciples, Zedar's appearance became similar to Aldur's, as if the centuries of involvment had stamped the features onto him.
Ce'Nedra thought that Zedar was Belgarath the first time she saw him, in Enchanters End Game.