The Boiler is a Xenomorph caste, first encountered by the Colonial Marines from the USS Sephora on LV-426. It possesses a unique form of attack, whereby it will forgo the typical melee or spitting attacks used by other Xenomorphs in favour of suicidally rupturing its own body, drenching nearby targets in huge quantities of acidic blood.
The Boiler first appeared in Aliens: Colonial Marines and was one of several new Xenomorph castes created for the game.
The body of the Boiler is horribly deformed, covered with boil-like pustules and lesions and being generally decrepit in appearance. Their exterior armor is excessively aged and fragile, leaving them quite vulnerable to attack, while the numerous deformities on their bodies restrict the Boiler's movement to an awkward upright hobble. Most notably, they apparently lack the ability to "see" like other Xenomorphs and are essentially blind, reacting instead to noise and vibrations in their environment. They spend much of their time crouched and immobile, resembling fossilized husks, but will awaken upon detecting nearby sounds and begin to search for the source. When Boilers move about, the pustules on their body tend to glow, although this glow diminishes when the creatures are at rest.It would seem that Boilers are prone to naturally dying from their afflictions as well, as many pustule-covered xenomorph carapaces can be found in the sewers of Hadley's Hope.
Boilers adopt a kamikaze attack strategy whereby they charge their enemies before causing themselves to self-destruct, suicidally exploding and showering their enemies in acid. It is not clear how such behaviour may have evolved, although it likely emerged as a result of their weak armour and extensive deformities, which would otherwise render them nearly useless in combat. While a single Boiler is rarely a threat to a well armed and prepared team, a swarm can overwhelm entire groups in no time at all, especially given how their suicidal attack can cover a significant radius. Before charging, a Boiler will let out a piercing scream.
Boilers have highly degraded senses, and are essentially "blind"; they are incapable of rationalizing their environment like other Xenomorphs and react only to sounds and vibrations. They will typically stumble around blindly and aimlessly, although will move more swiftly when detecting sound. Boilers seemingly have a very short attention span, as they quickly lose interest in hunting for a target once any noise they may have made has died down.
The Boiler can be acquired in multiplayer by awakening it in select locations after a certain amount of time has passed in a game. They cannot climb walls and they have very limited radar sight. However, they are extremely fast and have a very deadly suicide attack that can be triggered by holding down the attack button. Boilers will also explode if sufficiently damaged, and have an added ability to spit acid not seen in singleplayer, although its range is limited.
- The concept of diseased/decrepit Xenomorphs that attack by charging the player and suicidally exploding was originally considered for the the 2010 video game Aliens vs. Predator, as seen in concept art for that game.
- Assuming the ageing theory is true, the Boiler is similar to the Carrier Form of the infectious alien species in Halo known as the Flood. Both Boilers and Carrier Forms are evolved from combat castes that have become too damaged or useless to fight, and have changed to become volatile and explosive. As well as the banelings seen in StarCraft 2. Banelings evolve from Zerglings.
- One explanation often suggested by fans for the Boilers' creation is that they are some kind of mutation created by radiation from the Atmosphere Processing Plant explosion. Supporting this theory is that Boilers seem to be invalid soldiers, and only appear in the sewers of Hadley's Hope. However, the novelization of Aliens explains that the detonation of an Atmosphere Processor does not create any radioactive fallout. Additionally, Stasis Interrupted reveals that all Xenomorph specimens at the colony were destroyed when the Atmosphere Processing Plant exploded.
- One of the multiplayer class abilities of the Spitter allows it to explode when critically damaged, showering nearby enemies in acid like the Boiler.
- Another theory that could be drawn from the little evidence shown in the game could be that the various new Xenomorph types are in fact the results of various experiments Weyland-Yutani have done to the Xenomorphs, as we see Xenomorph bodies dismembered, and the various new Xenomorph types locked in cells in the Origin facility. In the case of the Boiler, it is possible that the Weyland-Yutani scientists tampered with the Xenomorph's regeneration ability, causing damaged Xenomorphs to mutate into their volatile form.
- The Boiler appears to initiate it's kamikaze explosion by clawing at itself. Specifically, it is seen to tear off it's own lower jaw.