Monster spawner

A monster spawner in a thing that forms part of the final boss which shoots spawn cubes that each generate a monster where they land. In MAP30 these cubes can also telefrag monsters and players occupying their corresponding landing spot. In other levels the new monster and whatever occupies its spawn spot become stuck together. When first seen by a monsters spawner, the user will hear "To win the game you must kill me, John Romero!", played distorted and backwards.

The spawner is not rendered graphically by the engine (nor has it a sprite assigned to it), and the only evidence of its presence to the user is that spawn cubes emerge from its location.

The spawn cubes from the spawner land at spawn spots placed around the map. The spawn spots are used in rotation, in the order they are defined in the THINGS lump in the WAD file. If there are no spawn spots, the spawn shooter still makes its "aroused" sound, but never shoots any cubes.

Monsters which may be spawned and their spawning probability:
 * Imp (50/255 chance)
 * Demon (40/255 chance)
 * Spectre (30/255 chance)
 * Cacodemon (30/255 chance)
 * Hell knight (24/255 chance)
 * Baron of hell (9/255 chance)
 * Mancubus (30/255 chance)
 * Arachnotron (20/255 chance)
 * Revenant (10/255 chance)
 * Pain elemental (10/255 chance)
 * Arch-vile (2/255 chance)

Additionally, the rate at which cubes are fired by the spawner in the "Hurt me plenty" or higher difficulties is twice that of the "I'm too young to die" and "Hey, not too rough" skill levels.

Appearance statistics
The IWADs contain the following numbers of spawn shooters per skill levels: