Slenderman

Slenderman is the first boss to appear in Slender Fortress and is one of the oldest bosses in Medic's Slender Fortress server.

Description
The Slender Man is said to be a supernatural creature who stalks people, usually children, before brutally traumatizing them until insanity and kidnapping them. It is unknown what he does with them afterwards, but he likely murders them or turns them into his mindless minions known as proxies. His name originates from his appearance, taking the form of an impossibly tall and thin man with a white featureless face and clad in a black suit.

The Slender Man originates from an internet legend/Creepypasta created by Victor Surge (Eric Knudsen) on the Something Awful forums, where he posted pictures of a tall stalker photoshopped into scenes of depressed children and claiming that the photographer was missing shortly after taking the photos. While he was fairly popular since his creation in 2008, he truly became a legend in 2012 due the famous Unity game Slender. Ever since, he's received uncountable appearances in several games, online stories, and cameos in famous mediums.

In Slender Fortress
The Slender Man is always sided with Kate, as he is her master. He is a standard teleporter who simply warps near the players before staring at them and sending loud blinding static into their screens. The more violent the static gets, the closer the Slender Man is to killing the player. Simply being in his sight causes the static, and looking directly at him causes the static to get more brutal. Touching him instantly kills you. He plays his unique gif from Slender: The Arrival upon killing the player. Getting out of the Slender Man's sight by hiding behind corners or objects is a very effective method to despawn him, making him a very easy boss, though he does grow more violent the more pages the player collects. The only thing you should look out for is him jumping out of corners and forcing you to touch him.

There is also two more variants of the Slender Man. Classic Slender Man, an older and much more violent version of the Slender Man, and Slendyclaws. Slendyclaws is a complete reskin of the Slender Man, being dressed in a red suit and Santa hat, completely replaces the Slender Man during December, and plays Christmas music instead of static. Slendyclaws still retains an unedited version of Kate with him.

All of the Slender Man/Kate maps have a special bossappear in their place every three rounds.

In Medic's Slender Fortress server
His stats in Glubbable's is completely unchanged, the only changes in him is that he isn't sided with Kate. His static in Hardcore and Insane rises very fast to the point it's almost impossible to outrun him when encountering him directly.

To make things even worse, Slenderman is paired with SCP-049 to tag along with. It'll be a problem if you are sandwiched by both of them. Unlike other servers, Slendyclaws and Classic Slenderman are not in Medic's Slender Fortress server.