Iron Crypticle

There are four "special" combat rooms that have a chance of spawning in each run, and there is an achievement for completing each one for the first time. In general, they're all a little harder than normal rooms, especially because they're darker and enemies don't stand out as much. The Graveyard and Catacombs are tough to tell apart because both room tiles kind of look like a tombstone (one is a catacomb door, though). The Infestation room tile is a bunch of bugs, and the Mud Pits room tile is a tentacle coming out of a hole. Simply complete each of these rooms once to unlock its associated achievement, and then I recommend just avoiding them if you can. Note that the Infestation and Mud Pits rooms don't typically show up until at least Floor 3, so bear that in mind.