To open this door you must dodge every name during the special thanks portion of the credits after a pacifist run.
Undertale the locked door in.
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.
First you have to get the key to sans room.
Save before talking to sans at the end then reload.
Don t worry though he gives you a nice bed and some food.
Nothing will open it not even the expensive mystery key item.
That key will open the back door.
Then snoop around there to find an another key that unlocks a hidden door behind the house.
Finally i got around to doing this.
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By the way there s two other keys that you might want to check out if you already haven t.
Glyde can be encountered in this room before fighting papyrus.
If you are defeated by papyrus instead of seeing a game over screen papyrus will capture you and lock you in the guest room.
Anyone that s started up undertale for the first time will encounter a mysterious locked door early in the game s snowdin area.
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.
After that you go into sans room and will find another key on a box.
Undertale is about a child who falls into an underworld filled with monsters.
There is no other way to enter this building.
Repeat a few times to get a key to his room.
To do that you have to save just before he judges you and then reload several times until he gives you the key.