So evil, so ingenious, so vicious, I wish I had thought of this one. This is best used when the surface is made of wood. A set of stairs leading to a mansion door would be a nice setting.
The trap is laid by having a section of floor cut out. A support beam, or several beams, holds this section of floor upright in its place from a burrowed out cave below, or even a set of tunnels. A small section of floor should be selected, perhaps 3′ in diameter.
When the victim steps up on the cut out platform a ghoul pulls the beam out from the supported floor piece, thus causing the PC to fall to their doom. The fall itself may only be 6′ or so and not even cause any damage. The 6 ghouls that wait below is another matter. Once again this one is not technically a trap, its just a support beam. Its not the floor’s fault someone pulled the beam.