Not in Retro Large
Not in FusionFall Retro.
The following article is about something that appeared after the Birthday Bash update in FusionFall, was removed prior to the build FusionFall Retro uses, or was otherwise excluded from FusionFall Retro by its developers. Thus, the content below will not appear in FusionFall Retro; this article only exists to archive part of the original game.

The Mystery Dungeon is an Adventure Time-themed dungeon that has no missions associated with it, only monsters that players can grind on to get exclusive items. The dungeon can be entered through the warp portal in Eternal Vistas graveyard, guarded by Reaper. The dungeon is covered with many "false walls." The hero can walk through them to enter hallways.
Confused Reaper

Inside the Mystery Dungeon

The only monsters are Crazy Metal Knights, which the player can defeat for a Golden Marauder set, a Blue Marauder set, a Green Marauder set, and the Crystal Gem Apple Grenade. If players can find Stanley the Watermelon (from Adventure Time), they can buy some cool collectable T-Shirts.

T-Shirt Princess Bubblegum Close-Up

One of the sweet Adventure Time shirts players can buy in the Mystery Dungeon