Dead Cells

There is a chance after each boss you fight for the bank entrance to spawn in the saferoom. It is a giant chest like the ones you find in the levels. When you are in the bank there are merchants on the different floors, one of which will be the mimic. There is no way to tell which one it is until you buy something from them. Once you buy an item from each merchant, if they don't change then move on to the next one. Once you buy an item from the mimic it will transform and attack you. Killing it gives you the achievement and an upgraded version of whatever item you tried to buy.