Where to Use the Mystery Key in Deltarune Chapter 5

Secret bosses have slowly become one of the biggest traditions in Deltarune, and Chapter 5 continues that trend with another optional challenge tucked away behind a locked Pink Door. To reach it, players first need to get the Mystery Key. This is a special Key Item purchased with Pink Coins collected throughout the Flower Kingdom. While buying the key is simple enough, actually finding where it is used can be easy to miss during a first playthrough. If you've already purchased the Mystery Key but aren't sure where it belongs, this guide explains exactly where to use it in Deltarune.