The Offspring creature from Alien Romulus, was a three-way hybrid between a Human, Xenomorph, and Engineer.
It stood at around 8 feet in height, with a gaunt, skeletal build, a very Engineer-looking head, face, and skin colour. It was seen to grow a long tail just like a Xenomorph. On the creature’s back, there can be seen the beginnings of four dorsal tubes, also like those of the Xenomorph XX121. It also has an inner mouth, similar to an Alien, as well as claws which it used to disable Andy with a single swipe.
It has deep, black eyes like an Engineer, and seemed to smile as it stalked and attacked people, giving it a very creepy and disturbing essence.
The Offspring was created from Z-01, or Prometheus fire, a variant of the Black Goo pathogen, harvested from Facehuggers by Weyland Yutani researchers aboard the Romulus module of the Renaissance space station.
The Black Goo was first seen in Prometheus, and then in Alien Covenant, a mutagenic substance which, when introduced to a living organism, attaches itself to it’s DNA, breaks it down, and reforms it into entirely new lifeforms; depending on the amount, the outcome can vary wildly, with the effects seemingly occurring at random.
In one case where a single drop was administered to Dr Holloway, his body began to slowly degrade before he was killed by Vickers to protect the ship from contamination, so we didn’t see the end result, but as he’d procreated with Dr Shaw after being infected, he passed it on to her. Creating inside her, the Trilobite, which was a squid-looking creature, which serves a similar purpose to a Facehugger once it has grown to full size. After removing it from herself, the Trilobite attacked and impregnated the Engineer, and birthed the Deacon, a creature similar to a Xenomorph.
When a large amount is used, like when David drops the entire payload onto an Engineer planet, it destroys them instantly, freezing them in place like victims of a volcanic eruption. As the pathogen spread through the wilderness, it created motes, which would release airborne particles of the pathogen, which when making contact with a Human, creates a Neomorph.
When the researchers on the Romulus extracted the Black Goo from the Facehuggers, they intended to use it an an evolutionary accelerant on Human beings, to better adapt them to life in colonial worlds, as they were thought to be to fragile to do so naturally.
Rook, a synthetic persuaded the group from Jacksons star that it could be used to heal injuries, and one of them, Kay, injected herself with it; but in actuality, the blend was not perfect, and it mutated her baby inside her into the Offspring, which she gave birth to.
It resembled a Human baby at first, but rapidly grew to adult size within a few minutes, and could be seen growing still.
The Offspring is born inside a pod which looks similar to an Ovomorph, and contains acid, like the Xenomorph blood.
Rain tries to dispose of it, but the acid melts through the floors, and she loses it; after a short while, the grown Offspring reappears, and kills Kay, and looks to be feeding on a gooey substance coming from her breast.
This is actually foreshadowed earlier in the film when they first enter the Romulus module. On the artwork on the walls, among a pile of bodies, there is a baby feeding from a dead woman.
After a brief chase, Rain kicks over the eggpod, spilling the acid out, melting a hole through the floor, and the Offspring falls through, before having a container dropped on it, falling into the planet ring.
If you were wondering where the connection to the Engineers comes from, and why a Human injecting themselves with Black goo would result in an Engineer hybrid; Humans were created by Engineers using the black goo, and share almost 100% the same DNA; so when broken down, it is essentially the same as Engineer DNA, so the hybrid looks like an Engineer.
The Black goo was either created or discovered by the Engineers thousands of years ago, but the exact origin of the substance isn’t yet fully understood.
The Engineers who used it, did so to both populate worlds with new life, and to destroy them as they saw fit; the ultimate act of playing God.
The Engineers themselves were pale humanoids, bigger than a Human, and just as, if not even more intelligent, and highly advanced both culturally, and scientifically.
They are known for using the black goo, and for their biomechanical technology, which blends the organic with machinery.
They created Human life on Earth, by one of their member sacrificing himself by drinking a container of black goo, which broke down his body, and repurposed his biomass into new life; which would evolve to become Homosapiens.
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