Mysterious Door and Dog Room

Mysterious Door and Dog Room covers the Snowdin secret cave players notice early but cannot open.

Where it is

South of a Snowdin ice puzzle area, a cave leads to a locked door marked with the Delta Rune. The Mystery Key item does not open it.

How it opens

Complete True Pacifist. During the long credits name list, dodge every Kickstarter backer name (do not let names turn yellow from hits). Start a new game afterward and return to the door — it should be open.

What is inside

A developer gag room featuring the Annoying Dog, props, and jokes about how the game was made. It is an Easter egg, not a required story beat.