SPOILER!
Haven't seen any videos on YouTube unlocking the closet door in Down in the Dungeon yet. Not going to show were the key location is until December 25th....but here is the easter egg opening the door in an unlisted video I just recorded. It took me almost an hour to find the key after @Jaybee commented there was one lmao. Enjoy and see if you can find the key (I looked everywhere!!!)