The Slender Man boss's skin is white, it wears a black coat, a blood red tie, and a faceless head, with blood on its hands. Since the 10.5.0 update, it is much taller than the Slender Zombie and has acquired six tentacles that are sprouted from its back.
Slender Man will walk near the player, dealing high damage to the player via swiping. Since the 10.5.0 update, its six tentacles will curve toward the player as it approaches. In the same update, it has also gained the ability to teleport, allowing it to effectively surprise and attack the player from behind. When teleporting, the screen experiences a flash similar to a television failure, with a quick shine of a light. This boss will continue to teleport next to you and then instantly attack you.
If you happen to be in front of this boss then it will just simply teleport behind you, but if you are trying to enter mazes or do complex maneuvers, this boss will most likely teleport to the side of you, on the left or the right. Upon defeating this boss the player will get the Double Barreled Shotgun (PG3D), one of the most powerful weapons in the campaign game mode.
- His appearance is a direct note to the Slenderman character created by Something Awful users.
- It is the only boss other than the Bug Boss & Admin that has the ability to teleport.
- The tentacles on Slender Man attack the player at different times before he performs his melee attack. This is different from the alternate attack mechanic of the Tortured Prisoner where the electricity and the melee deal damage at the same time.
- This is the only boss who attacks in milliseconds because the tentacles also do damage. This means that the player will take damage quicker but in smaller amounts.
- This and the Tortured Prisoner are the only Bosses that have an attack other than their main attack.
- Although he is slow, his teleportation allows him to cover some distance very fast.
- He is capable of killing low-level players in iron armor in a single shot, as his tentacles help him do extreme damage.
- This boss has one of the highest damage per second in the game.
- He originally looked like a normal Slender Zombie with red hands.
- There are many ways to get the Slender Man to teleport (and get stuck) in the mountains in the map. One way is to climb on top of the highest tree, another method is to use the Sword of Shadows, which can confuse the Slender Man.
- You can fight slender man without taking damage if you go to the forest of trees that are barren and dead looking and get on top of one of the trees. He will teleport onto the other trees around you, but can't reach you or teleport onto the tree you are standing on.
- A good tactic of defeating this boss is by using a high damaging close range weapon. Perhaps something like the Heavy Shocker or a Flamethrower type weapon. However, try to keep in mind that this boss can easily catch up with you and that you should do this tactic at your own risk.
- You could try to move backward and shoot the boss, but this is even riskier since you could easily run into a tree.
- If you are at low levels, then stay away from this boss at all times since this boss will deal massive amounts of damage to you.
- Never use melee weapons as the Slender Man attacks extremely quickly, unlike other bosses. Instead, use any ranged weapons if you wish to damage him with little chance of being damaged by that boss.
- Use the Singular grenade, then engage with a high DPH sniper. If you don't have the Singular grenade, walk around in circles whilst using a sniper or primary.
- Headshots are a must when engaging its users.