Xenomorph (Earth-5875)

"That which you know as xenomorphs are your salvation through destruction."

- - Gravemind

The xenomorphs, designated as LF.Xx.3273 by the forerunners (Latin Linguafoeda acheronsis meaning "foul tongue from Acheron"), and referred to as the parasite, the serpents, and the infection by the Covenant species, are a highly aggressive sapient endoparasitoid species that can quickly reproduce and grow through the infection or impregnation of other species with sufficient cognitive capabilities. With origins shrouded in mystery and uncertainty, xenomorphs reproduce by spreading its infection towards other organisms, hijacking their bodies and impregnating them with a Chestbuster, which in turns is converted into one of the species' many specialized forms. By impregnating other species, xenomorphs become increasingly more intelligent, and because of their frighteningly rapid growth rate, they are effectively unstoppable.

All xenomorphs share one single consciousness, which increases with each host it impregnates, eventually forming the ultimate xenomorph shared mind known as a Gravemind: a self-aware and incredibly intelligent entity which commands all other xenomorphs during outbreaks. Because of their group-like consciousness, the xenomorphs are better described as a single, trans-sapient macro-organism instead of individuals. When a Gravemind is not present, xenomorphs are only basically coordinated through the Feral Stage, where they seek to build more and more biomass through hives, leading to the creation of Proto-Keymind and Keyminds in the Coordinated Stage. Xenomorph's collective consciousness can only be described by humans as likening to a socialistic utopia, with each component of the species working to ensure their survival.

Biology
"You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility. You admire it. I admire its purity. A survivor...unclouded by conscience, remorse, or delusions of morality."

- - Ash and Joan Lambert Xenomorphs are primarily solitary predators, but they are fully capable and willing to adopt swarm tactics when larger groups are formed. Even though they are highly voracious and can easily kill their enemies, xenomorphs prefer to let their victims alive in order to be used as incubators for Facehugger Chestbusters or spores. Victims can also be taken to hives and their biomass used to generate a Proto-Keymind. If their targets are deemed unsuitable for either infection or impregnation, however, xenomorphs do not hesitate into quickly killing their chosen host.

They are extremely resilient, being capable of surviving under extreme conditions which range from -75 to +53 degrees Celsius, as well as underwater, effectively being able to withstand the varied environments of all known inhabited planets through the Milky Way galaxy. They are, however, able to proliferate and fare better on moist and humid environments, with artificial cold environments being implemented on important areas through the Halo Array and other forerunner installations intended to hinder xenomorphs. Xenomorphs will transform the environment surrounding them to better suit their needs, altering the atmosphere of a planet and converting into more of their biomass.

Anatomy and physiology
Xenomorphs will alter their physical appearance upon development or host, sporting different characteristics depending on the species they are infecting or impregnating; this ensures the adults are suited for the environment. This ability occurs at a cellular level under the xenomorph super cells, also known as "LF.Xx.3273" by the forerunners, and known as the DNA Reflex by xenoanthropologists. Most xenomorphs encountered in the modern era are gestated from the bodies of humans, sangheili, jiralhanae, unggoy, kig-yar, yanme'e and san'shyuum, and therefore they adopt bipedal stances, despite the existence of other variants that have been rarely found, from creatures and animals the xenomorphs have accumulated through Halo installations and unknown corners of the galaxy. Despite the differences of the host species, the xenomorphs still preserve core traits found in all variants.

Xenomorphs have a skeletal, biomechanical characteristics, which some theorize means they are an artificially created species, being either black, grey, or bronze-skinned, and no matter the host organism, all variants possess elongated, cylindrical skulls, with no facial features outside of their mouths. Their head possess a distinctive carapace, which can vary, from smooth and translucent to opaque, believed to be part of their maturation process. Because of the DNA Reflex, the flood can appear in both plantigrade and digitigrade forms, according to their host; for example, human, jiralhanae, unggoy and san'shyuum spawned xenomorphs will possess humanoid hind limbs, while flood spawned from sangheili, kig-yar, yanme'e and other animals, will possess hind legs.

The primary weapon of adult xenomorphs is the inner pharyngeal jaw, a piston-like tongue with a secondary set of jaws inside their mouth. It is powerful enough to break through bones and metal, and even energy shields. A common xenomorph killing strategy is restraining its victims with their hands and immobilize them before giving a precise jaw strike directly to the head, piercing the skull and penetrating the brain, known by UNSC as a "headbite". They also have segmented and bladed tails, which varies in size, from small to flat, which make deadly potent weapons capable of impaling and lifting sangheili and jiralhanae, as well as killing a mgalekgolo in two strikes. They possess dorsal tubes on their backs, consisting of two pairs, whose purpose is unknown.

Impregnation
To reproduce and grow their biomass, xenomorphs impregnate, infect, and assimilate other living beings, both sapient and non-sapient, and, despite their capability of using nearly all organisms as hosts, they take preference with the former, as the cerebral tissue of their victims allow them to develop advanced intelligence.

The initiation of a xenomorph outbreak begins with contact with either xenomorph spores or Facehuggers, the latter which are the most complex variant create by the species during their Feral Stage. After seeing a potential host, the Facehugger will jump to its victim's face, rasping clothing and flesh, whereupon it renders the victim unconscious and impregnates them with a Chestbuster. After two minutes, or several hours depending on the stage of the xenomorph, the Chestbuster violently erupts from the host's chest, killing them immediately, and rapidly matures into another variant, shedding its skin and growing, while replacing its cells with polarized silicon. Because of the DNA Reflex, the horizontal gene is transferred from the host to the xenomorph during the gestation period, making the adult xenomorph inherit basic physical traits of the host.

Impregnation is not limited to living beings, as deceased organisms can also become infected, as long as the body has not decomposed to the point of the nervous system deteriorating, Facehuggers can still inject Chestbusters to the body. Using the victim's biological content as a template, the adult xenomorph keeps the memories of its victim intact through isolating the cognitive capability of their brain.

Despite the high efficiency of Faceguggers, xenomorphs can also infect others through the spores, that, if inhaled, transform a host into an Aggressor or other variant, becoming the primary mean of spreading when an infection has reached the Coordinated Stage. They are tiny, and are deployed in a biosphere in large quantities, overwhelming unprotected targets.

Forms
Xenomorphs encompass a myriad of different forms, each one with their own unique abilities and roles within the xenomorph's never-ending development.

Spores
Spores are minuscule airborne xenomorph variants meant to quietly spread their infection through suffocation and anyone who inhales them.

Facehuggers
Facehuggers are the most basic xenomorph form, which encompass two known variants.

Gravemind Facehugger
Keymind Facehuggers are incredibly rare, being capable of laying a Keymind embryo within its host, alongside a second standard embryo in another host.

Abomination Facehugger
Praetorian Facehuggers, alongside Keymind Facehuggers, help in the direct creation of Keyminds through the implantation of Praetorian Chestbusters.