He is the Evil Overlord ruling the Great Tomb of Nazarick. Ainz, originally called Momonga, is a skeleton mage called a "Lich" and one of the most powerful people in the world who, while good with tactics and quick thinking, lacks foresight and often underestimates the behavior of his underlings. This has set the plot in motion that everyone wants to conquer the world for him, something he often worries about. Nicknamed "The Strongest Spellcaster, with the Appearance of a Skeleton" due to the aforementioned reasons.
Ainz also sometimes plays the role of "Momon", a black knight who fights with two huge swords, often with the purpose of collecting new information by doing undercover missions. Helping him in this endeavor is Narberal Gamma, assuming the name "Nabe" and pretending to be his colleague, rather than servant.
