Myster is a fan-made antagonist.
Its appearance consists of sepia, photo-negative, grayscale, and normal Mickey colors. There are multiple holes on its body, with the left shoe and arm missing. Its head has some distortions and is floating separate from the body, lacking any sort of pupils. The snout also seems raised, coupled with a gaping mouth. Oddly, the right shoe is still there despite not having its right leg.
Its Shade supposedly shares a similar appearance. It also has glowing ring-like eyes.
Myster starts in Character Prep 2, and slowly travels to the Staff Area, Meat Freezer, Lounge, then the Office. If it's directly in front of the desk, the player must shut off a camera. If it is further away, the player must hide. Failure to comply with the right mechanic will immediately cause it to kill the player.
Cutting the power/lights will have its Shade appear on occasion; it is completely harmless, and will not attack the player.
- Due to Myster's appearance, it's been mistaken for Corruptus. Despite some similarities, they are not connected characters.
- One of its voice lines consists of it humming. Said humming is a reversed version of the first few notes from the chorus of Steamboat Bill, famously featured in the 1928 Mickey Mouse cartoon Steamboat Willie.
- Its first jumpscare screech originated from a screaming frog.
- Myster was also originally one of the antagonists in the legacy version of the Five Nights at Treasure Island fan-game A Week at Castaway Cay. Due to it no longer fitting in terms of the improvements, however, it was ultimately cut from the game.
- Its behavior and jumpscare screeches, heard in older builds, were different.
- Myster's Shade is directly based on the Shades seen in the official Five Nights at Treasure Island.