Arshag was a tattooed Grolim priest, of the Temple at Calida, posing as a wizard who was able to conjure the image of Nahaz, the Demon Lord. He was associated with Harakan and had been in contact with Nahaz.
Initially, Arshag had his Karandese crowd enthralled, but was debunked by Belgarath, posing as a wizard. Arshag's illusion had no substance. Belgarath summoned a very convincing illusion of The King of Hell. When Arshag tried to run, Garion stopped Arshag and then performed a wide crossing punch which felled Arshag.
Arshag revealed that he had several wizard teachers and when each one ceased to be able to teach him, Arshag killed each one and sought a new teacher.
Arshag was in the service of Harakan and Nahaz to take Cthrag Sardius and cause the Prophecies to cease to exist and to rule over the world.
Arshag also convinced young Karandese women to be impregnated by the Demon Lord Nahaz.
Eventually Polgara pronounced a terrible curse and compulsion on Arshag such that he was invincible but that nobody would ever believe him again. Arshag would be faced with constant ridicule and derision for the rest of his life and he would be driven out wherever he went, to wander the word as a rootless vagabond.