Lichdom

He does. The Overlord series states he is an undead sorcerer which typically is the definition of a lich. However Ains specifically says Clementine is mostly right when she calls him one. The only thing is that in the lore of the MMO where Momonga had his beginning, his race was a skeleton, a race that was never alive to begin with.  In my opinion being undead, self-aware and a sorcerer, he still qualifies.