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.

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. Additionally, in more recent first-person shooters enemies are intelligent enough not to fire if a fellow enemy is in the way.

Monsters capable of causing infighting


Monsters that fire inaccurate, continuous firing or blast-damage weapons (Troopers, Sergeants, Chaingunners, Wolfenstein SS, Mancubi, Arachnotrons, Spider Masterminds and Cyberdemons) are particularly susceptible to making friendly fire on other monsters and therefore being attacked.

Any monster that fires projectiles (the previous plus Imps, Cacodemons, Revenants, Hell Knights and Barons of Hell) can be maneuvered into doing this.

Lost Souls, being projectiles themselves, can damage other monsters found on their attacking path, causing their retaliation.

Demons and Spectres, being melee attackers only, will not fire any projectile susceptible of hit other monsters.

Monster unable of causing infighting
Arch-Viles attacks will never make the hit monsters return fire, nor the creatures hurt by its blast damage will retaliate.

Pain Elementals and the final boss, whose attack are carried by proxy monsters, cannot provoke monster infight.

Commander Keens, who are harmless, cannot bother other creatures.

However, all of them can still be hurt if they get in the way of monsters trying to hit a third party, and therefore initiate infighting. Exceptions to this rule are Commander Keens, who simply cannot attack anyone, and the final boss, whose tight vulnerable window is unreacheable to any rocket-firing monster (Cyberdemons) in any official level yet designed, and whose algorithm doesn't allow it to order monsters it spawns to attack anyone but the player.

Tactics
Monster infighting is an important tactic in levels where 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 with a large group of mixed monsters is to run in a circle a couple of times around the group, which will almost certainly start an infighting due to stray projectiles. This is allowed by the unusual monsters/player high speed ratio that, contrary to many other first-person shooters, Doom, Heretic and Hexen features, who grants the player the ability to swing among monsters groups with ease.

Difficulties


The above strategy is complicated by two factors.

First, monsters of the same species are immune to their own projectile weapons. Hell Knights and Baron of Hell, as well as Demons and Spectres, are of a single species in this sense, and cannot hurt one another with plasma (except indirectly, e.g. destroying a close barrel). Although a given species of monster is immune to its own type of attack, they still employ it with its customary frequency while targeting one another. The immunity also means that those individuals from the same species cannot actually inflict damage no matter how many times they attack each other (except indirectly, e.g. by destroying additional barrels). An Arch-Vile is subject to blast damage from the attacks of other Arch-Viles, but does not retaliate.

The second caveat has to do with Doom's monster AI. Monsters resurrected by Arch-Viles or spawned by the final boss 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 (either if it was successfull or not) and then turn back to the player or its new attacker, although the monster that the Lost Soul had damaged 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.