Psychic / Glitch
Mega Mewssingno X or Y glitches the game. Does critical damage and opponent pokemon's shape and form is laggy.
Screen turns dark for a whole minute. Only Mega Mewssingno, its team, and its trainer can see.
Mewssingno was created after Mewtwo met Missingno. After a huge battle, Mewtwo eventually fused with it, creating a much more powerful pokemon. Eventually, it got a mega stone after it defeated Arceus (really, its that powerful, but don't worry Arceus didn't die- I MEAN FAINT RIGHT KEEP IT KID FRIENDLY) and thus, this powerful monster was dropped into existance.