Monster infighting

In Doom, Heretic and Hexen, many of the enemy monsters can be tricked into fighting each other, or infighting. This can usually be accomplished by moving so that the player and two monsters are colinear. Since a monster does not have the intelligence to cease fire in the event that a fellow monster blocks his line of sight, said fellow monster will be subject to friendly fire. Combat between the two monsters will soon follow, and continues until one participant either dies or is distracted by a third party's attacks. In Doom, monsters will also retaliate when damaged indirectly by exploding barrels. Monsters that fire inaccurate or blast-damage weapons (notably Sergeants, Chaingunners, and Mancubi) are particularly susceptible to making friendly fire on other monsters and therefore being attacked, but any monster that fires projectiles (Imps, Cacodemons, Revenants, Barons of Hell) can be maneuvered into doing this. Demons, on the other hand, being melee attackers, can easily take damage from monsters behind them and therefore initiate infighting. However, the Pain Elementals and Arch-Viles attacks will never make the hit monsters return fire, even though they can still be hurt or killed if they get in the way of monsters trying to hit the player.

Monster infighting is an important tactic in levels where health and ammunition are very scarce, such as those of Alien Vendetta, or when recording speedruns, especially in a restrictive category such as UV Tyson. Causing many monsters to destroy or seriously damage one another greatly reduces the burden on the player's resources. The easiest way to do this is to run around a group of mixed monsters, which is almost certain to start infighting due to stray projectiles. Another method is to deliberately line up an approaching monster so that it is directly between the player and a projectile-throwing monster in the background.

The above strategy is complicated by two additional factors. First, monsters of the same species are immune to their own projectile weapons. (Barons and Hell Knights are of a single "species" in this sense, and cannot hurt one another with plasma. An Arch-Vile is subject to blast damage from the fireballs of other Arch-Viles, but does not retaliate.)  One such monster attacks another only if damaged indirectly with a barrel.

Although a given species of demon is immune to its own type of projectile attack, they still employ it with its customary frequency while targeting one another. The immunity also means that certain species, such as Mancubi, cannot actually inflict damage no matter how many times they attack each other (except indirectly, e.g. by destroying additional barrels).

The second caveat has to do with Doom's monster AI. Lost Souls ejected by Pain Elementals, monsters resurrected by Arch-Viles, and monsters spawned by the Icon of Sin immediately attack the player, not their creator's target. In addition, no monster is permitted to target an Arch-Vile, and Lost Souls damaged by other monsters "forget" their targets after a single counterattack and turn back to the player (although fortunately for the player, the monster that the Lost Soul had attacked does not forget the Lost Soul). (Certain source ports include more complex AI, and allow the user to decide how aggressive monsters should be during infighting.)

Monster infighting has become a novel aspect of Doom and its progeny. Indeed, most games do not give the enemies the ability to hurt each other, much less the ability to retaliate. In more recent first-person shooters, however, enemies are intelligent enough not to fire if a fellow enemy is in the way.