from the Slender Man Mythos
Costume 1: You know, the business suit and everything.
Costume 2: A shadowy all-black version of himself
The Slender Man is a mysterious tall figure from an unknown realm. For centuries, it has been stalking, kidnapping and slaughtering human beings, most of whom were young children, which are his favorite meal. Now it made it's way into Outworld, where the Mortal Kombat tournament takes place. Upon discovering the tournament, it has agreed to participate in it. It's purpose for entering the tournament, of course, is to kill any combatant it faces in this ancient death match and he might even find new people to kill and eat or whatever he does to them.
*Finger Swift: Slender Man swings his hand with all his fingers erect and smacks down his opponent.
*Portal Uppercut: He disappears into the ground and uppercuts his opponent from the ground next to him/her.
*Tentacle Trouble: Similar to Arakune's move from BlazBlue; He summons a portal to the ground under his opponent and a tentacle starts will either squiggle around him/her or grab him/her and starts smashing him/her everywhere.
*Stomp: He stretches his leg while close to his opponent and stomps on him/her.
*Dimensional Shots: He opens a portal close to his opponent and it starts shooting little dark orbs at him/her
Slender Man raises his right hand with two fingers sticking out and stabs his opponent in the eye with them. Then he grabs his opponent with his left hand and squeezes him/her, shattering both arm bones and the ribs, turns his hand upside down and slams his opponent to the ground on his/her head, cracking the cranium.
*Dimensional Murder: Slender Man grabs his opponent, opens up a portal and drags himself with the opponent and begans to brutalize him/her inside that portal, causing blood to fly out of the portal. 5 seconds later, he comes out of the portal holding his mutilated opponent with his right hand and grinning at the screen with bloody teeth, implying that he was eating his opponent.
*Tentacle Slaughter: Slender Man lets out his tentacles and picks up his opponent on the limbs with them. Then he rips the limbs off his opponent with said tentacles.
Baby Slender Man grabs a nearby doll with both his hands and swallows it whole. Then it burps and then shows it's teeth.
A portal opens up on the floor and the Slender Man crawls out of the portal that afterwards closes.
The Slender Man stares at the screen and releases his tentacles while making creepy noises.
Slender Man's possible realm (complete with the Smile.jpg based monster)
-Stage Fatality: You can throw your opponent at the Smile.jpg creature and it eats him/her.
Despite winning the tournament, the Slender Man felt the need to take away more lives, as if Shao Kahn and some other kombatants it faced weren't enough. It began to invade all other realms and kill inhabitants, starting with the Shaolin Order from Earthrealm. It raided the monastery and started killing and eating all the Shaolin monks in the process. It was then stopped by Liu Kang, who challenged the Slender Man to Mortal Kombat. Unfortunately, the Slender Man was too invincible for Liu Kang to handle, as It ripped Liu Kang in half during the battle and then continued the unnatural massacre in the Shaolin monastery. No realm has been safe from the Slender Man ever since.