MX is the main antagonist of the Creepypasta game Mario '85, series
He's an ancient spirit-like demonic who inhabited a copy of Super Mario Bros. and disguised himself as a monstrous eyeless version of Mario himself in order to lure unsuspecting players/victims into the game and play with for all eternity. He is Lucas' arch-nemesis, his first and only victim, of whom he killed and pulled his soul into his digital hellish dimension, forcing him into the body of Luigi.