Mysterious Door

The  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 found in multiple areas of the game, is surrounded by glowing mushrooms. Glyde can be encountered in this room before fighting Papyrus.

Opening the Door
To open the door, the protagonist must avoid 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' Gauntlet of Deadly Terror, a "text-to-speech" converter that tells the player that the dog somehow programmed the game, and a dog-shaped husk of Dog Residue.

Dogroom
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