Editing Monster infighting

[[File:Friendly fire.png|right|thumb|A [[shotgun guy]] killing another on the [[E1M1: Hangar (Doom)|first level]] of [[Doom]]]]

In the [[Doom games]], [[Heretic]], and [[Hexen]], many of the enemy [[monster]]s can be tricked into fighting each other, a phenomenon which is generally called '''monster infighting'''. This can usually be accomplished by maneuvering into a position where one monster is in the line of fire of another. Since a monster does not have the intelligence to cease fire in the event that a fellow monster blocks his line of sight, it will attack without regard, possibly resulting in {{wp|friendly fire}}. The damaged monster may then [[Monster behavior#Choosing and switching targets|target]] the attacker, where combat between the two monsters will soon follow and continue until one participant either dies or is distracted by a third party's attacks. In the Doom games monsters will also retaliate when damaged indirectly by [[Barrel|exploding barrels]].

[[File:Heretic_Infighting.png|thumb|right|Infighting does not just occur in Doom. Here, in [[Heretic]], an [[undead warrior]] has managed to anger a [[golem]].]]

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 {{wp|first-person shooter}}s, enemies are either intelligent enough not to fire if a fellow enemy is in the way, or to avoid the line of fire of other enemies.

== Monsters capable of causing infighting ==
Monsters that have inaccurate, repeating, or blast-damage attacks ([[zombiemen]], [[shotgun guy]]s, [[heavy weapon dude]]s, [[Wolfenstein SS]] officers, [[mancubi]], [[arachnotron]]s, [[spiderdemon]]s and [[cyberdemon]]s) are particularly likely to cause infighting.

Any monster that fires projectiles (the previous plus [[imp]]s, [[cacodemon]]s, [[revenant]]s, [[Hell knight]]s, and [[Baron of Hell|barons of Hell]]) can be maneuvered into doing this. [[Lost soul]]s, being projectiles themselves, can damage other monsters found on their attacking path, likewise causing these to retaliate. 

== Monsters unable to cause infighting ==
Monsters damaged by an [[arch-vile]]'s attack will never retaliate because there is an exception in the [[Doom source code|source code]] that prevents other monsters from targeting them. This does not stop arch-viles themselves from targeting and damaging other monsters, though. Arch-viles are easy to distract, as, unlike other monsters, their lack of a [[Monster behavior#Targeting threshold|targeting threshold]] means they will always switch target when hit.

[[Pain elemental]]s, whose attack is carried by proxy monsters, cannot provoke other monsters, but will still turn on attackers if provoked.

[[Baron of Hell|Barons of Hell]] and [[Hell knight]]s have an immunity to each other's projectiles coded onto their mobj IDs. (This was not kept in [[PlayStation]]/[[Sega Saturn]] Doom and [[Doom 64]].)

Hanged [[Commander Keen]]s do not have an attack mode of any sort, so they can neither provoke other monsters nor retaliate when they are damaged by projectiles.

Also, in [[Heretic]] and [[Hexen]], no monster will retaliate against "boss" opponents who accidentally strike them outside of the mass infighting detailed below. Code-wise, this is handled by a flag attached to the boss [[mobj]]s.

===Melee monsters and infighting===
Monsters that are [[Melee attack|melee]] attackers cannot initiate monster infighting by mistake. This concerns enemies such as Doom's [[demon]]s and [[spectre]]s; Heretic's [[gargoyle]]s, [[golem]]s and [[sabreclaw]]s; Hexen's [[ettin]]s, [[centaur]]s and [[stalker]]s; and so on. The [[Doom source code|source code]] reveals that melee attacks are impossible to misdirect; the damage is directly inflicted on the target if it is within range. Aiming is not involved and therefore hitting another target by mistake is impossible.

However, [[Hank Leukart]]'s [[Official Doom FAQ]] states that misdirected melee attacks do happen and can trigger infighting. Before [[Versions of Doom and Doom II#v1.5|v1.5]] of the engine, {{c|A_SargAttack}} (the demon attack routine) used a short-range hitscan attack. Demons and spectres retain this behavior in [[Atari Jaguar]]-based console ports, including those for the PlayStation and Saturn, leaving infighting between them possible. From v1.5 onward, the hitscan routine is replaced by a simple distance check and direct damage.

[[Raven Software]] added a feature to [[Heretic]] and [[Hexen]] which causes all monsters within sight of the [[player]] to begin attacking randomly chosen targets when the player dies in a single-player game. Monsters will fight with any living thing within their sight when this happens, including members of their own species and normally ignored creatures such as [[boss]]es (i.e anything shootable, including [[Voodoo doll]]s, in which case the player' screen will, despite being dead, still flash red each time the doll is struck). However, monsters remain unable to hurt other monsters of their own type with their [[projectile]]s even in this case, as can be viewed in {{c|p_map.c}} line 331, where there is no check as to whether or not the player is dead or alive when rejecting same-species projectile hits.

== Tactics ==
Monster infighting is an important tactic in levels where [[Ammo|ammunition]] is very scarce, such as those of [[Alien Vendetta]] or when recording [[speedrun]]s, 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 several times around the group, which will almost certainly start an infight due to stray projectiles. This is allowed by the unusually high player-versus-monsters speed ratio in [[Doom engine]] games that grants the player the ability to maneuver around groups of monsters with more ease than in other first person shooter games.

Arguably one of the most prominent examples of this strategy can occur in ''[[Doom II]]'' ''[[MAP20: Gotcha! (Doom II)|MAP20: Gotcha!]]''. This level happens to feature two of the most powerful enemies in the game (a [[spiderdemon]] and a [[cyberdemon]]) in the same room. Each is easily capable of dispatching the player, but with quick maneuvering the player can actually cause one to accidentally hit the other, starting a lengthy and entertaining fight that inevitably results in one killing the other, but not before being severely injured, making it much easier for the player to finish the job.

Another advantage is that causing infighting will not alert peacefully waiting monsters like using a weapon does. For example, it may be wise to wake up a [[Final boss|demon-spitter]] as late as possible.

Although [[arch-vile]]s can only single-sidedly infight another monster, this can still be useful for the player: first, the opponent is being damaged. The only caveat is that once the enemy is dead, it may instantly be resurrected to its full strength, which may be worse than before the arch-vile attack. Second, the arch-vile will follow the other monster instead of the player. A cyberdemon accidentally hitting an arch-vile often leads to the arch-vile moving close to the cyberdemon and into its line of fire, and likely finished off by consecutive cyberdemon rockets targeted at the player.

== Difficulties ==
[[File:Baronsinfighting-Dawnofthedead-e1m8.png|thumb|right|256px|Two [[Baron of Hell|barons of Hell]] infighting in E1M8 of [[Jan Van der Veken]]'s [[Dawn of the Dead]]]]
[[File:Doom 64 MAP31 Baron Knight Infighting.png|thumb|right|256px|A [[baron of Hell]] and a [[Hell knight]] infighting in [[MAP31: In The Void (Doom 64)|MAP31: In The Void]] of [[Doom 64]]. The two monsters will infight with eachother normally in console ports featuring both enemies.]]

The strategy of provoking monster infighting is complicated by a few factors.

Primarily, monsters of any species are immune to their own kind's projectile attack (but not [[hitscan]] attacks), unless a barrel is hit and damages another of their kind. Hell knights and barons of Hell are of a single species in this sense, and cannot hurt one another with their green fireballs except indirectly by destroying a barrel, though this is not the case in the console versions. 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 by destroying additional barrels or if they get close enough to each other to use a non-projectile attack, like claws or a bite. Arch-viles are subject to peripheral blast damage from the attacks of other arch-viles on nearby monsters, but do not retaliate because they cannot be targeted by any monster.

Also, monsters (aside from arch-viles as described above) will not always immediately retaliate against the most recent entity that damaged them; instead, a monster will hold a grudge for some time to pursue its current target in lieu of new targets. When a monster acquires a new target its "threshold" variable will be set to 100. The variable will decrement by 1 each time the monster takes a step (this is tracked by distance traveled, not literal steps in their animation). A monster will not switch targets so long as its threshold is greater than zero and its current target is still alive. 

Additional factors that lessen monster infighting have to do with some characteristics of the Doom engine's [[Monster behavior|monster AI]]. Monsters resurrected by arch-viles or spawned by the final boss immediately target the player, not their creator's target. In addition, lost souls damaged by other monsters [[Lost soul target amnesia|forget]] their targets after a single counterattack (whether successful or not) and then turn back to the player, although the monster that the lost soul had damaged does not forget the flaming skull's offense. Certain [[source port]]s include various AI options, and allow the user to decide how monsters should behave in respect to monster infighting.

There is another AI related showstopper for monster infighting: sometimes, one of the monsters that can be tricked into infighting is located on another height level. For example, if a Hell knight on the street is hit by an imp behind a first-floor window. In this case, [[projectile]] attacks may only be successful from far away. Unfortunately, monsters often first walk up toward the opponent, making it impossible to ever reach the other party. Sometimes, the player can help them by moving into their way.

== Player interactions ==
During an ongoing infight, the player can attack and damage the infighting monsters. However, the infighting monsters will typically continue with the infighting and ignore the player's attacks.

During an infight, the player can naturally be injured by missed attacks and [[blast damage]] or simply by walking into the monsters' line of fire.

== Other games ==
===Monster infighting in Doom 3===
Monster infighting can occur in [[Doom 3]] as well. It does not happen very often, though, because of the significantly smaller monster numbers and due to the lack of room where the player could lure the monsters into each others line of fire. Infighting in Doom 3 is further complicated by the monsters following a certain hierarchy. For example, an [[Imp (Doom 3)|imp]] will not attack a [[Hell knight (Doom 3)|Hell knight]] because of the Hell knight's higher "rank". 

True infighting in [[Doom 3]] is technically impossible without mods since the monster hierarchy essentially consists of only two classes: An "upper" class (Hell knights, revenants) and a "lower" class (imps, Z-Secs). Only monsters in the upper class are allowed to attack other monsters, and even then, they will only attack monsters of the lower class. The lower class is simply incapable of fighting back. Thus, any "infighting" that occurs is very one-sided.

Note: According to the developer's comments on the files found in game, monsters of equal rank are supposed to be able to fight each other. However, at least in the PC/MAC version of the game, this does not occur for whatever reason.

====Ranking System====
This information is based on the code files found in the game:

Rank 0: zombies, lost souls

Rank 1: imps, Z-Secs, commandos, pinky demons, maggots, wraiths, ticks, trites, cherubs 

Rank 2: Hell knights, revenants, arch-viles, cacodemons, mancubi, vagary, Guardian of Hell, Sabaoth 

Rank 3: cyberdemon

Note: It seems strange that commandos, who are considered quite powerful opponents, would be relegated to the lower class. This could be due to them being formerly human.

===Doom (2016)===
Monsters in [[Doom (2016)]] follow a complex set of rules for infighting which involves the existence of factions (roughly corresponding to UAC-created or purely demonic) and a global anger level between those factions which is tracked during all combat. This makes non-scripted instances of infighting relatively rare, but more commonly observable between certain types of monsters. For example, [[Revenant (Doom 2016)|revenants]] and [[Hell knight (Doom 2016)|Hell knights]] are more likely to infight due to the area damage caused by the latter monster's fist-pounding leap attack. Though monsters are able to damage each other in this game, they often will not finish each other off, as monsters do not consider each other valid targets while they are in a staggered state.

Scripted instances of infighting can also occur, where the map's designer has set monsters' targets to other monsters on the map at spawn. Several instances of this can be observed in [[Kadingir Sanctum]], where for example a number of [[Imp (Doom 2016)|imps]] will destroy a horde of [[unwilling]] as soon as the player enters their proximity.

===Doom Eternal===
Infighting in Doom Eternal is a random and constantly present phenomenon. When monsters are outside of a certain aggravation distance of the [[Doom Slayer]], they will look for other targets to engage in mock combat with - monsters are strictly not allowed to damage one another in this game. While completely cosmetic, this can still occasionally be to the player's advantage as it can keep monsters distracted. Some levels in the game, such as the [[ARC Complex]], position monsters in unreachable but visible areas (such as inside of offices in tall skyscrapers) where they infight perpetually, meant as a set piece to make the environment more lively.

== See also ==
* [[Barrel suicide]]

== Source ==
* {{wikipedia|title=Monster infighting}}

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[[Category:Doom engine]]
[[Category:Monsters]]
[[Category:Tactics]]