Unknown Specimen 3, referred to as Spooper in the game files, is one of the new specimens encountered in Endless Mode.
Spooper's normal form resembles a child wearing a ghost costume. The sheet is white and featureless with two eyeholes and he is wearing black sneakers. The sheet will become tattered and soaked with blood the more Spooper is struck with the Axe, and on closer inspection, there is an eye in his chest area.
As the chase starts, Spooper's parasite form resembles a pink fleshy ball that floats through the air. A single eye is in the center of its body and it has two arms that claw at the air while it chases the player.
Spooper will first be found standing in front of the exit door of any given room where he might spawn. He turns to face the player extremely slowly. The door can not be entered when he is blocking it, forcing the player to use the axe to destroy him.
Upon entering another room, Spooper will appear again blocking the exit, bloodied from the previous encounter. The player must continue to destroy him with the axe and move to the next room, Spooper becoming more injured each time. After destroying him in his most injured state, entering the next room will begin his chase sequence.
The grimy rooms the chase takes place in are filled with a caustic air which will drain the player's health slowly. During the chase, Spooper occasionally flashes in the room and a pink monster spawns, but doesn't appear to do any harm. Sometimes, the player will fall to the ground and choke up blood, inhibiting movement and depleting a substantial amount of health.
When he kills you, a text will appear:
"Beyond that door is another world.
A world made of dreams...
A world where all your fantasies have come true.
A world full of everyone's sick fantasies.
A world lusting to include you."
"SPOOPER AMB", Unknown Specimen 3's chase theme
- In Arcade machine mentioning Spooky's backstory, there's an image of person in a ghost costume which looks similar to Spooper's.
- Because the doors aren't disabled when Spooper appears, in some rooms it's possible to go around it and enter the next room. Thus Spooper will appear in the state it was in during the last room and continue to do so until it's hit or the player enters a save point.
- Sometimes, when the player vomits, there is a chance that a fetus-like creature covered in blood will appear.
- Unknown Specimen 3 and Specimen 5 are the only specimens that can change the textures of the walls in normal rooms.
The mutant form of Spooper will now appear more rarely and relatively smaller. Instead, Spooper's bloodied form will occasionally flash on the screen, slowly turning towards the player before disappearing.
Its chase music will now start as soon as Spooper first blocks the door instead of only after being "killed".
Spooper's original form will now teleport to the door the nearest to the player, preventing them from bypassing it in rooms with multiple doors.
Spooper's damage output has been significantly reduced, although it now disables healing, putting its new output comparable to its original version. Also, the puddles of blood produced by the player will now slow them down similarily to Specimen 2's ink puddle.
- As of The Halloween Update, it can no longer make the player vomit if there is another specimen/monster around.
- This was the most requested feature by those who played Endless Mode.