Overlord Momus is a fictional being created by Arfoire who is claimed to be supposedly stronger than the goddesses. It is said that he is the one who created the monsters and that those who defeat them will face judgment.
He is rumored to be so strong that even that original goddess was barely able to seal him away. The current goddesses had no choice but to escape to the human world in order to escape danger. Evidence of this is supposed to be the fact that all of the goddesses, except one, came down to their lands at once; that single goddess being Purple Heart.
They stood no chance against the Overlord, ran away, and let Celestia get ravaged. The Overlord is said to have a messenger who spreads the word of Momus and punishes those who kill the monsters prowling about in the human world.
Throughout the game, Overlord Momus is mentioned numerous times by Heretics who fear the Overlord's Messenger, as well as the Evangelist. The Heretics encountered in caves are usually old men who carry around discs given to them by the Overlord's messenger that summon monsters. The Heretics are loyal to the Overlord and his messenger due to the belief that spreading monsters throughout Gamindustri will prevent the world from being destroyed by him.
The Evangelist, Conversation, uses Overlord Momus to convince Yvoire, the Archbishop of Leanbox, to try and kill Neptune so Planeptune's source of protection will be severed and people will migrate to the other lands, thus increasing the other goddesses' shares. At the end of the Leanbox Key Fragment arc, Neptune realizes that the Evangelist and the Messenger are in fact the same person.
Upon Histoire rescue after all the Key Fragments have been collected, she reveals that Overlord Momus was nothing more than a complete falsity fabricated by Arfoire who intended to use it as way to shake the people's faith in the goddesses. She further explains that Evangelist and the Messenger where disguises she had taken to follow through with her schemes on each land, and that the Arfoire's they had fought up until now were just mirrors of her true self.
Hyperdimension Neptunia Re;Birth1
While a good chunk of the story of this being remains unchanged from the original, upon the theft of Purple Heart's powers, Arfoire uses Histoire's tome to actually make Momus real, basing it off of people's imagination and fear of it. After Compa is able to steal back Histoire, Momus is sicked upon the heroes.
Despite being defeated, it simply revives. This is due to people's fear of it causing it to be able to remanifest itself. Thanks to MarvelousAQL using a flash bomb, the heroes are able to escape it's wrath.
After the CPU successfully obtained three of the four legendary weapons, Arfoire and Momus arrive on Planeptune to destroy it, but Neptune and her team arrive to fight it. They manage to beat it, if barely, and it was thanks to the other three CPU spreading word of Momus being nothing but fake, but it is still able to revive. However, thanks to the belief in the CPU, Momus is finally defeated by the CPU.