Monster spawner

A monster spawner is 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 (50/256 = 19.53% chance)
 * Demon (40/256 = 15.63% chance)
 * Spectre (30/256 = 11.72% chance)
 * Cacodemon (30/256 = 11.72% chance)
 * Mancubus (30/256 = 11.72% chance)
 * Hell Knight (24/256 = 9.38% chance)
 * Revenant (20/256 = 7.81% chance)
 * Arachnotron (10/256 = 3.91% chance)
 * Baron of Hell (10/256 = 3.91% chance)
 * Pain Elemental (10/256 = 3.91% chance)
 * Arch-vile (2/256 = 0.78% chance)

Note: In Vanilla Doom, the Demon actually has a higher spawn chance than the Imp due to the way the pseudorandom number generator is constructed and a lower spawn chance than the Arch-vile.

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: