Xenomorph (Earth-2915)

"One single xenomorph egg can destroy a species."

- Rtas 'Vadum

The xenomorph, designated LF.Xx.3273 by the forerunners, xenomorph XX121 by the Weyland-Yutani Corporation, and referred to as the parasite by the Covenant, is a species of highly aggressive endoparasitoid extraterrestrial species that reproduce and grow by copulating sentient lifeforms of sufficient biomass and cognitive capability. The xenomorphs are vicious predatory creatures with no higher goals than the propagation of their species and the destruction of any life that could pose a threat to them. The creatures are known for their potent acidic blood and their pharyngeal jaws, although their biological life cycle, in which their offspring are implanted inside living hosts before erupting violently from their chests, is in many ways their signature aspect. The xenomorphs were responsible for consuming most of the sentient lifeforms in the galaxy - including the vast majority of forerunners - during the Forerunner-Xenomorph War in ancient past, prompting the activation of the galaxy-sterilizing Halo Array in 97,445 BCE.

All xenomorph forms share a single consciousness, which becomes progressively more intelligent and sophisticated with every sentient host that is assimilated. When enough hosts have been infected, the xenomorphs consciousness becomes a Gravemind - a self-aware, highly intelligent entity that strategically commands all xenomorph forms in an outbreak. Due to this group consciousness, the xenomorph species as a whole is more accurately described as a single, transsentient macro-organism rather than a collection of individuals directed by one or more controlling beings. In the absence of a Gravemind, the xenomorphs possesses only basic coordination; they are incapable of forming complex strategies, and focus solely on attacking and infecting nearby sentient organisms. This stage of a xenomorph outbreak is known as the Feral Stage. At this stage, xenomorphs seek to create an assembly of biomass, calcium, and nervous system reserves, which leads to the creation of a xenomorph hive and the beginnings of a proto-Gravemind. The nature of the xenomorphs' collective consciousness is similar to that of a hive mind, the xenomorph act as a unified entity, with no individuality that is inherent to other species; each vessel of a xenomorph works tirelessly to aid in the propagation of their species.

The xenomorph presents a highly variable and unconventional threat in combat, as it can impregnate and mutate dead or captured opponents into a myriad of deadly forms, effectively growing stronger as it weakens enemy forces. They are widely considered to be the greatest threat to the existence of life (or, more accurately, biodiversity) in the Milky Way galaxy.

Source

 * https://www.halopedia.org/Flood
 * https://avp.fandom.com/wiki/Xenomorph_XX121_(Alien)