The “Alien” (also referred to as “Xenomorph XX121”) is a fictional endoparasitoid extraterrestrial species that is the eponymous antagonist of the Alien film series. The species made its debut in the film Alien (1979), and reappeared in the sequels Aliens (1986), Alien 3 (1992), and Alien Resurrection (1997), as well as the crossover franchise Alien vs. Predator (2004) and Aliens vs. Predator: Requiem (2007). A similar creature of a slightly different design named the “Deacon” makes a brief appearance in the Ridley Scott film Prometheus (2012). Two species, one being a different but similar creature called a “Neomorph” and the other being a variation of the Xenomorph XX121 species appear in the sequel to Prometheus, Alien: Covenant (2017), which reveals the potential origin of the species. In addition, the Alien appears in various literature and video game spin-offs from the franchises.
Unlike many other extraterrestrial races in science fiction, the Aliens are not “tool-makers”; they lack a technological civilization and are predatory creatures with no higher goals than the propagation of their species and the ultimate destruction of lifeforms that could pose a threat to them; indeed, it is unclear whether the Aliens are even sapient or not. Like wasps or termites, Aliens are eusocial, with a single fertile queen breeding a caste of warriors, workers, or other specialists strains. The Aliens’ biological life cycle involves traumatic implantation of endoparasitoid larvae inside living hosts; these “chestburster” larvae erupt from the host’s body after a short incubation period, rapidly mature from juvenile into adulthood within hours, and seek out more hosts for implantation.