The Mysterious Door is an area found in Snowdin Forest, south of the right side of the room with the icy X's and O's puzzle. The door, which bears the Delta Rune, is surrounded by glowing mushrooms and floating particles. Glyde can be encountered in this room before fighting Papyrus.
Opening the Door
To open the door, the protagonist must dodge all the names of the Undertale Kickstarter backers in the end credits of the True Pacifist Route. There are other ways to get open the door, such as editing the SAVE data.
Beyond the door lies the "developer room" which includes the Annoying Dog who represents Toby Fox, a computer open to bad posts that the dog has made, a jigsaw puzzle, the rope that was probably used in Papyrus's Gauntlet of Deadly Terror, a "text-to-speech" converter telling the player that the dog somehow programmed the game, and a dog-shaped husk of Dog Residue.
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- The dog made up of residue that then falls apart may be a reference to page 2832 of the webcomic, "Homestuck."
- The piece of fabric that the Annoying Dog is lying on states that "the dog needs to 'patch' the fabric." This is most likely referring to the actual game itself, as Toby Fox needs to patch the game up.
- If the Mystery Key is used when facing the door, a click is heard, but the door does not open.
- The music that plays in the room,
mus_dogroom.ogg, cannot be found in the Undertale Soundtrack. It can be heard here:
- This is very likely a reference to the Silent Hill 2 dog ending.
- ↑ You used the Mystery Key. The door's lock clicks... ... as you fail to fit the key into it. Nothing happened.