“The ancient one, the destroyer of souls” (3:332). It is an ancient, evil spirit that is a necromancer. Its portrayal to Alex was a giant, insect like body with evil, green eyes and long, dripping fangs, though this was in an illusion world, so it is unclear if that is its true form.
Ages ago, he invaded Thraxon but was trapped inside a cave by the dwarf king Set. The Order of Malgor knew of its existence, and the dwarf wizard Nethrom of the city of Neplee learned of the existence and sought out the cave. Once it possessed Nethrom, it began to use its power to subject the dwarfs of Neplee. It raised many undead animals, including the Hellerash.
Mog’s cave or prison was protected by undead animals on the mountainside, a hydra at the entrance, jewel bats further on, undead dwarfs who are only bound to dig for Mog, and a golem made of stone.
It was destroyed by Alex on his third adventure.