His lack of arcane mastery wasn't from a lack of effort. No, he studied long and hard in the early years of his life His only reward was sporadic success and the sensation of the weave withering at his touch. Casting even low level spells is like trying to hold on to a crumbling ledge.
His grandfather perhaps was one of the only ones gracious enough to see past the politics and potential status problems of his condition. That is why he gifted Conlyn with the bone handled rapier known as ::Mageblood::
By the decree of the Council of mages all eleven of these such weapons were ordered destroyed. Taruathier knew what danger awaited his grandson, living in a magedom where crimes against many non-mages were rarely brought before the courts. He lied to the council, and documented the destruction of another magical blade in it's stead.
Taruathier knew, if Conlyn was to survive he needed the vengeful blade.