M-DEUS is an optional secret boss from Tokyo Mirage Sessions ♯FE that can only be fought in a New Game+. He is the true form of Nieg, the Mirage that trades Incenses to Itsuki in exchange for Detritus in the Urahara Arena. King says that he is a Mage Dragon and an ally of Medeus. He can fought by being spoken to at the Arena and selecting the option, "I want to challenge you to a battle." King then takes on his true form of M-DEUS, and the battle begins.
He is an extra-powerful boss patterned after Medeus with a level of 99, making him the most challenging fight in the game on all difficulty levels.
M-DEUS's name is a direct play on Medeus. "Deus" is also the Latin word for "god."