Thalor the Black was a pack mage that could fire out lightning bolts. He could not be killed unless his special artifact, the eye was destroyed. Eventually Connacht destroyed the eye and ended the pack mage's life during the events of Myth 3 the Wolf Age.
Given that GURPS states that Thalor's Eye is lost--but Connacht destroyed the artifact the "Eye of Thalor," it is safe to assume that there were two of these legendary items created by the Myrkridian lord himself. Like a pair of eyes, Thalor probably had one on the Myth planet and the other on his home world. With the Summoner summoning the Myrkridia, Thalor could have possibly wrecked havoc once again upon the forces of Light.