Sanatorium is the 1st map in the Slenderman's Shadow series, released in August, 2012.
Like the game it is based on, Slender, the goal is to collect 8 pages scattered in a derelict asylum, instead of a forest. The pages are located about the facility, but unlike Slender and future installments of the series, the pages do not move or randomize to different parts of the map.
Your character is armed with nothing but a flashlight, similar to the original Slender. The flashlight is capable of illuminating anything in front of you, and is controlled with the camera controls. The player is armed with no other items, and has no defense until the player can collect the 8 pages. Once that happens, the player can take out a pistol and attempt to shoot the Slender Man, but this has been removed in the newer versions.
The player must collect all 8 pages before being captured by the Slender Man. Fortunately the pages stand out very brightly and are very easily visible. 
In the Unity remake, pages do randomize and have changed to a brown colour, sometimes difficult to see.
- This is the first in the Slenderman's Shadow Series.
- The gun used in version .9.5 was a golden Desert Eagle, but was then changed to what looks like a USP.45.
- If the player does shoot Slender Man, there is a short after note in which the player says that they become Slender Man.
- The after note's text is this: "I did it. I had escaped the nightmare that was hunting me. Or so it seemed. It was then that I discovered Slenderman does not let his victims escape. He only chooses to. I have taken his physical body, and in doing so, he has taken my mind. I am Slenderman."
- This is the first map to be indoors.
- First Slenderman's Shadow map to collect pages.
- This is the first time the player plays as a man, unlike in the first and second Slender games.
- Slender Man's model is different from Parsec's Slender. He has visible hands, longer arms and legs.
- There is not a sniper in Sanatorium.
- Slender Man can apparently move in this map, as he actually slides inside rooms and in front of doorways, creating a new type of scare for players.
- This is the first Slenderman's Shadow map (and possibly the only Slender map) where you can shoot the Slender Man.