The Ballroom is a room located in the Grand Hall of the Mansion within The Nightmare. You require an Ancient Key to access this room, as it is locked behind gold chains.
Summary
The Ballroom is a room with a spinning floor (it will not move you in any way) filled with mysterious dancing robots who will not react to anything you do and cannot take damage nor deal it. It is very dark in this room and enemies can appear from any side of you.
The room is also filled with a mysterious power that affects you as soon as you enter it. You will slowly begin to move slower and slower until you're unable to move at all. There doesn't seem to be a way to fully negate this effect so you must cross the room quickly or you may be overwhelmed when the enemies being to come after you. The Funky Driftlander may be helpful to cross the room if you find yourself stopped in place as the slowness effect won't stop your weapons from moving you.
The exit is on the other side of the room, which connects to a box seat room in the Theater where you can meet the Tickermaster.