The Deacon is a very specific Xenomorph predecessor born through a gruesome and unfortunate chain of events involving Dr. Elizabeth Shaw and Dr. Charlie Holloway in the 2012 Alien movie Prometheus. The Deacon is not a common alien variety and hasn't occurred again outside of the one specimen in Prometheus.
The Deacon has one of the most interesting and unique life cycles of all the Xenomorph or Xenomorph adjacent beings in the Alien franchise. Its unique life started when David, a Synthetic, spiked Holloway's drink with Black Goo, infecting him with the mutagen.
Holloway then had intercourse with his partner, Dr. Shaw, infecting her with a Black Goo mutated alien fetus. Once born. the creature then managed to capture the Engineer that was loose upon the ship, thus creating the Deacon as we know it.
Although Holloway was the first one to make the incredibly stupid decision to take his helmet off on an alien planet that they knew nothing about, that is, surprisingly, not how he managed to get himself infected by the Black Goo. That was all David.
David approached Holloway when he was alone, offering him a drink that he subtly dipped his finger in. What Holloway didn't know was that David had a tiny drop of Black Goo on the tip of his finger. The amount was small, but it was enough to cause devastating effects to both Holloway and Shaw.
Holloway's demise was a slow and painful one that the crew first mistook as an illness. The Black Goo eventually began transforming him into an unrecognizable being before he baited Meredith Vickers into exterminating him with a flamethrower when he realized he would likely not survive the contagion.
Before his gruesome and untimely demise, however, Holloway and Dr. Shaw had intercourse, leading to the next phase of the Deacon's life cycle.
Trilobite
When Shaw and Holloway had intercourse, the Black Goo contagion was transferred into Shaw's uterus via Holloway. It then proceeded to develop rapidly into a horrifying alien fetus dubbed the Trilobite.
Dr. Shaw did manage to surgically remove the Trilobite before it burst out of her using a surgical pod and modified instructions. This saved her life but also released the terrifying being at the same time, and despite Shaw's attempts to trap the alien in the pod, it eventually escaped.
The Trilobite was able to continue its metamorphosis into a massive tentacle monster or proto-Facehugger. After escaping from the pod, the massive tentacled Trilobite captured an Engineer, successfully implanting the embryo that would become the Deacon.
The Trilobite is the Facehugger phase of the Deacon's life cycle, but it differs greatly from the Xenomorph variety, mainly in terms of its physical form. The Trilobite is absolutely massive compared to its smaller and more compact Facehugger cousins. With its greater size comes greater strength. Enough to take down a fighting Engineer, even.
The Trilobite relies more on its pure strength and power to take down its victims, unlike the Facehugger, which uses a much more silent and stealthy approach.
Deacon
The Deacon is the Drone step of the Deacon's life cycle and the point where it is the most similar to its Xenomorph successors. Unlike the modern Xenomorph, however, the Deacon emerges from the host fully grown instead of a small worm-like creature that then grows into its final form.
The Deacon has light-colored skin, unlike the black Xenomorph. This is likely due to its Engineer host, who also had light blue skin. The Deacon is also more biological in its formation, lacking any of the biomechanical features of the modern-day Xenomorph.
It still has an elongated head, although the Deacon's comes to a sharp point, and the iconic second set of jaws. The Deacon's do not protrude out as much as a Xenomorph's second jaw, however.
The Fate of the Deacon
The Deacon is only shown for about ten seconds in the final scene of Prometheus. It is never revisited in any other Alien movie, and it's quite likely that it won't be. As an Alien movie buff, that ten-second clip is probably all you will ever see of the Deacon. It was, however, revisited in the Alien comics.
InPrometheus: Fire and Stone - Omega, the fate of the Deacon is revealed, and it is more obscure than any of us could have imagined. Instead of finding a host and perpetuating the species, the Black Goo caused the Deacon to continue to mutate until it eventually became such a monstrosity that it lost its ability to move or exist in any meaningful way. Its uncontrollable growth eventually rendered it a grotesque meat mountain, existing on LV-223 as a disturbing monument to the unpredictable nature of the Black Goo.
Deacon Appearances
The Deacon exists as a solitary example of its kind, appearing only in Prometheus-related media. You can find a list of all the media containing the Deacon below: