Anyone that’s started up Undertale for the first time will encounter a mysterious locked door early in the game’s Snowdin area. Nothing will open it, not even the expensive Mystery Key item. Learn how to get inside with the quick guide below.

Undertale is just full of mysteries, but this was plagued fans for a long time. Down the path east from Snowdin Village, there’s a long cavern that eventually ends with a seemingly impassable door. Don’t worry, there’s nothing story-related hidden here — no new dungeons or characters to meet, but completionist and super-fans might want to stop by for a visit.

See exactly why we loved this unique JRPG revival on The Escapist’s Undertale review, then check out our ‘Best of 2015’ list to see why Undertale deserves its spot.

How to Enter the Mystery Room

The optional ‘Mystery Door’ is a locked door in Snowdin Forest. It can be accessed just before the Papyrus boss battle. There’s a path leading south with a cliff — the dark cavern holes feature glowing eyes. Walk east to enter the cave that leads north to the Mystery Door.

• To open the locked door, you must earn the True Pacifist ending.
• During the ending credits, you must dodge the Kickstarter credits. You can’t hit and turn a single name yellow.
• Start a new game and reach the locked door. It will now be open.

What’s behind the locked secret door? It’s a Developer Room Easter egg!

What’s Inside the Developer Room

The Developer Room is a literal Developer Room — there’s a computer with silly posts, a text-to-speech converter, and clues that the little white dog is the actual developer behind Undertale.

There’s a unique item here too; the Dog Residue is a unique item that spawns an endless supply of additional Dog Residue or Dog Salad items when used. This can be used for gold farming later, if you’re into that sort of thing.

There’s also a Jigsaw Puzzle and a length of rope. Other than the annoying dog, there’s no real purpose to the real. It’s just a fun location to explore.

How to Get the True Pacifist Ending

Getting the True Pacifist Ending isn’t easy. We can’t cover every single boss battle, but we can give you the broad strokes.

• Gain no EXP. By the end of the game, you’ll need to have 0 EXP. If an opponent dies in battle, their heart will shatter. Reload a previous save if this happens.
• Make friends with Papyrus, Undyne, and Alphys. Alphys can only be made a friend after completing a single Pacifist run — after the final boss, if you gained no EXP, you’ll get the option to reload and try again. This time you’ll get the option to make friends with Alphys.
• Make sure to SAVE Monster Kid while escaping from Undyne. In Hotland, pour a cup of water on Undyne after she passes out.
• Complete the True Lab area in Alphys’ home, then continue on to the final boss battle.

Those are the broad strokes. Keep checking The Escapist for a more complete True Pacifist ending guide coming soon.

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Kevin Thielenhaus is a freelance writer for The Escapist. Find him on Twitter here.