Ghost monster

A ghost monster is a monster with unusual ghost-like properties. This is caused by a rare bug that occurs when the monster's corpse becomes a small pool of gore by being crushed under a door or ceiling and is then resurrected by an arch-vile. The ghost monster looks normal and still behaves like before being killed, except that it retains some of the properties of the pool of gore it had become. When moving, these properties allow it to pass through walls to sectors that are equal to or lesser in height in respect to the one it occupies. As far as combat is concerned, the ghost monster can usually only be harmed by splash damage, monster melee attacks, arch-vile attacks, and telefragging. Nonetheless, since its near-immunity to some attacks occurs because it cannot be aimed at, it is possible to harm it by hitting a specific, very narrow area at the base of its axis with a regular projectile attack, although this is very difficult to achieve.

Some PWADs have used the ghost monster phenomenon as a feature, such as Dark Side of the Mind, The Waterfront, Requiem's MAP23: Hatred, Icarus: Alien Vanguard's MAP24: The Haunting, Unholy Realms's MAP06: Sewage Entrypoint, Hell Revealed's MAP26: Afterlife, Hell to Pay's MAP14: The Habitat Deck and MAP22: Vile Temple, and Plutonia 2's MAP32: Go 4 It.

As with many Doom engine bugs, this one is fixed in many source ports, including Boom; while Doomsday retains it fully, other ports, such as ZDoom, automatically enable it for maps widely known to utilize the bug, namely the above examples, but otherwise prevents the bug from occurring. In MBF, it depends on whether the setting is enabled.

Technical
The following code creates small pools of gore: P_SetMobjState (thing, S_GIBS);

thing->flags &= ~MF_SOLID; thing->height = 0; thing->radius = 0;

The parameters are not restored upon resurrection, so all such pools of blood are potential ghosts if the original monster has a resurrection sequence (a set of sprite pointers usually containing the frames of the death of the monster in reverse). Most monsters have one, with the ones that cannot be resurrected by the arch-vile (lost soul, cyberdemon, spiderdemon and another arch-vile) being exceptions.

Demo

 * [[Media:RESRPAIN.WAD|A level demonstrating how a pain elemental can be resurrected, despite normally never leaving a body behind when it dies, as a ghost monster]] (file info). Step on button 1 to activate the crusher and wait for the pain elemental to die, and then step on button 2 to stop the crusher when it is near the ceiling. After that, step on button 3 to release the arch-vile and see the effect in action.

All-ghosts effect
In extremely rare cases, a memory overflow can occur which causes all things, including players and monsters, to become ghosts. In deathmatch play, this phenomenon is termed the DM no-clipping bug. One such way to cause this is when a hitscan attack, or a BFG tracer crosses over more than 128 linedefs/things at once, causing an intercept overflow. Such an overflow can be produced, somewhat awkwardly but quite reliably, for example, by shooting a hitscan weapon over a large amount of corpses and pickups all lined-up in a row.

This bug can happen in all Doom engine games except Strife, where a fix was implemented to stop progression of tracers when the intercepts array is full. In versions of Doom up to 1.2, and in Heretic and Hexen, it is also possible for line-of-sight checks to trigger the glitch, as a tracer-like algorithm is used for line-of-sight checking in those games which utilizes the same intercept array.