The Ghost Monster glitch is a famous Doom II glitch.
A ghost monster is a monster that is crushed and resurrected by an Archvile. They are immune to are all attacks and can go through walls, doors and even the level's boundaries. To make a ghost monster, you need only an Archvile and any crushed monster's corpse. Lead the Archvile to the monster's corpse, and with luck, it should be resurrected. This will result in the ghost monster. This glitch should work on any level with an Archvile, although in some cases you must be on skill level 3 or higher.
It is impossible to kill ghost monsters with most normal weapons. However, there are a few ways to deal with them. One might shoot a rocket on a wall or the floor; splash damage will hurt ghost monsters. This means that explosive barrels should harm them as well. In addition, other monsters' attacks will still hurt the ghost monsters. Telefragging also kills them. Telefragging is when a player teleports "into" a monster. The game handles this by instantly killing whatever is teleported onto.
There are a few exceptions to the rule. Lost souls cannot be resurrected as they leave no corpse. Archviles, Spider Masterminds, and Cyberdemons cannot be resurrected either. The Pain Elemental has a specific respawn sequence, and yet in most cases leaves no corpse. The last part of the death animation points to frame 0 (blank) so there is nothing to resurrect, unless they are crushed, which means they are only resurrectable as ghosts.
Ghost monsters can be resurrected an infinite amount of times. Leaving wandering ghost monsters does not count towards your kill total as they are already dead to begin with.
In addition, an extremely rare bug can occur called the "all-ghosts bug" has been known to occur. This is due to a memory overflow. In deathmatch, this phenomenon is termed the "DM noclipping bug." Every existing thing turns into ghost mode. This includes you, all the monsters left in the level (including those who normally can't be turned into ghosts) and non-living things that can't move (like scenery items).
- Doom Wiki for their help
- Ledmeister for his help
- GameFAQS for hosting Led's ghost monster FAQ
http://www.gamefaqs.com/computer/doswin/file/197142/8512 - Ledmeister's FAQ