Prometheus and Alien: Covenant introduced a strange black substance to the Alien franchise, but its true purpose has never actually been revealed. As the first prequel to 1979's Alien, Prometheus started laying the groundwork for what would eventually become the design for the terrifying Xenomorphs. Instead of the Xenomorph threat in Prometheus, however, an unusual black substance was introduced that posed a different, but equally scary, threat to the human explorers. Though it's never revealed where the substance came from, it's heavily implied that it was created by the alien Engineers, but its purpose remains unknown.
The Engineers formed the crux of the story told during Prometheus, with the alien species being revealed to have been the creators of humankind. Despite this, they seemed to harbor a hatred for humanity and wanted to use the black substance to wipe out life on Earth. The black substance seems to play a huge part in the story unfolding in the Alien prequels, so it's curious that it hasn't ever been explained, but two separate Reddit theories might be the answer to uncovering some of these mysteries.
The Engineers Could Have Made The Black Goo
The events of Prometheus heavily imply that the Engineers made the black substance, despite it never actually being confirmed. Prometheus suggests that the black goo seems to be a living substance, with the capability to alter other lifeforms, such as in the way it transformed the worm in the large room filled with canisters, or how it turned both Charlie Holloway and Fifield into disfigured creatures. This kind of transformation is reminiscent of what happens to the Xenomorphs themselves, since they take on features from the host body, which has led to a Reddit theory suggesting that the black substance could come directly from Xenomorph DNA.
While Prometheus suggests that the Xenomorphs evolved due to the Engineers and David's meddling, the Reddit theory instead poses that the Engineers found the Xenomorphs already in their full form. Seeing the Xenomorphs as the perfect killing machine, the Engineers could have wanted to harness their power, breaking them down to their DNA to enable them to use the subsequent substance for a variety of purposes, such as drinking it to break down their bodies, or using it to wipe out whole civilizations. The Xenomorphs are parasitic, burying their embryos inside living hosts, and the black substance is parasitic too, so this theory could very likely be the case.
Why Did Prometheus’ Engineer Drink The Black Goo?
The opening moments of Prometheus feature several beautiful shots of an ancient Earth before the Engineers land, one of which drinks a cup of the same black substance that the human explorers on the Prometheus would find later on. The substance spreads throughout the Engineer's entire body and seems to break down every cell, causing the Engineer to fall into a river, where what appears to be human DNA begins to form. This is an interesting new creation myth introduced in Prometheus that shows just some of the power of the black substance, and a separate Reddit theory also has an answer as to why this happens.
It's evident from the events of Prometheus that the black substance turns any living thing it touches into a more aggressive and violent creature, which could point to an answer as to why the Engineer drinks the substance. This idea would therefore make humanity the more aggressive version of the Engineers, perhaps created as test subjects for their various experiments, or in fact as experiments themselves. It's possible that the Engineers simply wanted to populate barren worlds with their own DNA, but then there would be no purpose to the black substance. This theory suggests they had more nefarious ideas for humankind.
Why Did The Engineers Want To Wipe Out Humanity?
Despite the Alien prequels promising to answer burning questions from the original Alien films, Prometheus and Alien: Covenant left more mysteries in their wake, including why the Engineers wanted to populate the Earth, and consequently why they wanted to wipe out humanity. The sheer devastation that could have been caused by the black substance on Earth was glimpsed during a flashback scene in Alien: Covenant, which showed David's elimination of the Engineers on their homeworld, pushing his obsession with the substance and the Xenomorphs. The first Reddit theory does suggest a reason for the Engineers wanting to do that to humanity.
While some believe that the Engineers had simply forgotten about humanity, and so when humans came looking for answers, they wanted to eliminate their experiment, the Reddit theory suggests otherwise. Instead, it theorizes that the Engineers wanted to wipe out humanity because of humanity's innate hubris, meaning that, given the opportunity, humans would use the Xenomorphs as a weapon against a plethora of other species. This would make humans a threat to the Engineers, especially now that humans have actually found them. Until a sequel to Prometheus and Alien: Covenant is released, this could be the best explanation for the Engineers' deadly plans.