Slender Man is a creature created on the Something Awful forums. It was made popular through a web series called "Marble Hornets", which inspired the popular game Slender, the Eight Pages and its sequels.
Slender Man is a tall, skinny monster that stalks its victims for quite some time, driving them insane before either brutally killing them or abducting them. It normally wears a black suit and tie and has no facial features. It can grow tendrils from its back and extra arms. As well, Slender Man possesses inhuman strength, which it uses to mutilate its victims by either ripping them apart or disemboweling them.
In M.U.G.E.N, Slender Man has been made by various creators.
Israel Guevara's version
This version of Slender Man is a strange character in general, having only two attacks and a bunch of specials, which all use power. SlenderMan's sprites are custom made. One special shows a creepy picture of Slender Man that is invulnerable to everything except the F1 key, and if the moves hits the opponent, it will slowly drain their health to zero. This character comes with its own music.
This character does not have any Special or Hyper Attacks.