Slender Mann is a BLK Spy with no face, He is similar to the Creepypasta, SlenderMan.
It is unknown where Slender Mann came from, but it is theorized that Nightmare Medic summoned him to Stalk and Kill all teams.
Another theory (created by a Wikia contributor) says that Slender Mann was a result of a freak teleporting accident, like with Solgineer, but it happened in a Freak Fortress 2 server. It happened when a BLK Spy was sapping a teleporter exit when Slender Man used the teleporter entrance connecting to where the Spy was. That Spy was unfortunately standing on top of the exit when Slendy came through, and when Slender Man rematerialized, they both fused into a BLK Spy with no face and white skin, creating Slender Mann.
Slender Mann was created by Youtuber TF2 Freak Professional, Saris Khan.