The Circus Watch is a consumable item found in the Nightmare. The only known way to acquire one is as a potential reward from the Nightmare Boss.
In appearance, the Circus Watch is nothing more than a simple brass pocket watch attached to a brass chain.
Upon entering the Nightmare Boss battle, the Circus Watch is consumed to bring the player(s) straight to the Circus phase of the fight, effectively skipping the initial parts of the battle.
The watch is present in the Itempedia of the Nightmare, and it is capable of stacking allowing players to bring as many watches as they wish to the battle.