The Deacon — also known as an Ultramorph — is an artificial Xenomorph-like species created by the Mala'kak (Space Jockeys) to be used as a weapon, which turns around and backfires horribly. It bears many resemblances to the Xenomorph; however, it possesses more human-like teeth and has an erect bipedal posture. Its head shape is also unlike that of Xenomorphs, and it seems to emerge from its host using its pointed skull as opposed to simply biting or punching through the ribcage.
It should be noted that the Deacon and its constituent castes are NOT the same species of Xenomorph encountered on LV-426; rather, LV-223 was a facility housing prototypes of this sub-species. Additionally, this does not count into effect the evolutionary time and gap between 2093 (Prometheus) and 2122 (Alien), which is a 30 year gap, not long enough for it to have evolved properly (though by the unique way by which the Xenomorph evolves it is even possible). However, each known infestation of Xenomorphs also seems to be somewhat different than the others, indicating some level of special variation presumably on-set by genetics leeched from hosts and their overall environment. The Xenomorphs encountered in 2122 and again in 2179 were believed to have been in hibernation for several millennia earlier.
The life cycle of the Deacon is similar, but still remarkably unique when compared to the Xenomorphs. Some examples of this are that its head is elongated but ends in a point. It lacks a tail but instead of a mouth-like tongue (the double jaw), it literally has another mouth within its first one which slides forward more like a goblin shark. However, it does resemble the Xenomorph by its reptilian/insectoid appearance and exoskeleton (and presumably the endoskeleton a Xenomorph also possesses). It also stands on two legs like some Xenomorph can.
A Deacon is released into a host by a Trilobite. The Trilobite seen in the movie was gestated as a fetus within Elizabeth Shaw's womb, where it grew with frightening speed. It was removed via Cesarean birth before it has fully grown and developed, yet it survived the makeshift abortion; after Shaw leaves the area, she returns to see that although she removed the creature from her body it has grown to be a giant starfish-like creature. It can be reasoned that since there have been many other deacons before the one seen in the movie, there must a shorter way to make a trilobite, such as a full grown deacon laying eggs much like a xenomorph queen. The Trilobite has multiple tentacle-like legs and coloring that resembles a Facehugger. The Trilobite soon infects the Space Jockey with a tube-like organ that injects an egg/fetus through the Jockey's mouth. This is also behavior of a Facehugger. It even dies after infection like a Facehugger, indicating that the two are very likely related, though it has not been confirmed that this is so.
The deacon egg is orally injected by the Trilobite. After only a couple hours, the Deacon is born. It completely bursts from the Space Jockey, not just from its chest, but its entire front side, using its head as a makeshift egg tooth.