First you have to get the key to sans room.
Undertale the locked door in.
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.
To do that you have to save just before he judges you and then reload several times until he gives you the key.
The door which bears the delta rune is surrounded by glowing mushrooms and floating particles.
If you are defeated by papyrus instead of seeing a game over screen papyrus will capture you and lock you in the guest room.
That key will open the back door.
There is no other way to enter this building.
Then snoop around there to find an another key that unlocks a hidden door behind the house.
Anyone that s started up undertale for the first time will encounter a mysterious locked door early in the game s snowdin area.
Repeat a few times to get a key to his room.
Save before talking to sans at the end then reload.
Undertale by indie developer toby fox is a video game for pc ps4 vita and switch.
Finally i got around to doing this.
By the way there s two other keys that you might want to check out if you already haven t.
Nothing will open it not even the expensive mystery key item.
Glyde can be encountered in this room before fighting papyrus.
Don t worry though he gives you a nice bed and some food.
We re an unofficial community of fans and a showcase for undertale fanwork.
To open this door you must dodge every name during the special thanks portion of the credits after a pacifist run.
In the nintendo switch version examining the door leading to mad mew mew in the dog shrine before it is unlocked yields a message saying that the door is locked by a mysterious seal as an ominous piano jingle is heard.
Come join the fun.