The Guillotine World is found by going through the bleeding wall near the beds in the Number World. It is significantly small and is filled with Lunatic Toriningen and closets. Interacting with the Guillotine will give you the Severed Head effect. Using the Bicycle, Stoplight, or Triangle Kerchief effect makes it easier to avoid the Toriningen and obtain the Severed Head effect. It is possible to escape this world back to the Bed Room in the Number World by entering one of the wardrobes in the Guillotine World.
If the sane Toriningen in the Number World is stabbed before entering the Guillotine room, then Madotsuki will instead go into the Big Guillotine World, which is the same as the Guillotine World except that it is larger and it contains six lunatic Toriningen instead of three.