I saw this question about how to open it, and I wondered what is inside it.
Has anyone seen this door? What exactly could be behind the mysterious door?!
I tried opening it but couldn't. Even tried lighting up the mushrooms with different sequences.
Perhaps it is supposed to open in the second play through?
Or maybe, you need to buy the key from those 2 girls [Bratty and Catty] who were selling trash?
Seeing the tag of Undertale, fills me with determination.
Best Answer
Behind the Mysterious Door lies a joke "developer" room. It contains only a few objects, each of which can be interacted with to show some text. The Undertale Wikia has a list of all of the text that the room contains.
Here's what the room looks like: