Carnag is a manifestation of Mira's escaped shaping power.
Many decades after the High King stole Mira's shaping power, the power escaped him and formed itself into Carnag. It then set about destroying towns in Sambria, though it did not kill any people. It was bound to Mira. It was made of debris collected from its rampaging.
Self-aware and in some control of itself, it was nevertheless guided and partly controlled by a woman named Quima, who is a member of some secret organization that has the ability to alter the shaping power itself (called shapecraft).