A0-3959x.91 – 15 (commonly referred to as black liquid) is a lethal biological compound with mutagenic qualities and is held in sealed containers by the Engineers as a way of creating and altering life.
The black liquid had been categorized by Weyland-Yutani giving it the designation A0-3959x.91 – 15.
In Prometheus, Elizabeth Shaw and her crew explored the Alien Temple where thousands of urns of the liquid was discovered. Due to the crew being in the Ampule Room and Temple changed the atmospheric conditions and as David touched one of the urns it shortly began to ooze out black liquid and at the same time the murals in the Temple began to alter. David manages to take a smaller urn of A0-3959x.91 – 15 out of the Temple which he later opened to discover it contained vials of the mutagen.
After he and Holloway share an alcoholic drink which was poisoned with a drop of the liquid. Sometime later Holloway discovers that he is infected and begins to be agitated so the rest of the crew bring him back aboard USCSS Prometheus. As he returns his skin continues turning green and his blood veins black, since the liquid can enter the bloodstream and other parts. Holloway is then torched by Vickers at his behest before he mutates any more.
After Holloway was killed, it was later discovered that the Engineers had changed their minds about the fate of humanity and decided to use the liquid to kill the humans. However, the Engineers failed to recognise the potency of mutagenic compound and an exposure affected them, leaving only one Engineer who was able to save himself by entering hyper-sleep in a sarcophagus. When the last Engineer was awakened by David and Weyland, the Engineer killed Weyland and decapitated David. The Engineer had a preset course as Earth as it's destination and planned to use the weaponized A0-3959x.91 – 15 to destroy human life. However, the Engineer's mission was stopped by Captain Janek and his crew, who crashed the USCSS Prometheus into the launching Derelict spacecraft.
A0-3959x.91 – 15 was also responsible for the creation of the Trilobite creature which was the result of Holloway having "sexual-relations" with Shaw while he was infected; impregnating Shaw with the Trilobite. The Trilobite later attacked the Last Engineer as he attacked Shaw in the crashed Escape Craft and was impregnated with the creature called the Deacon.
A0-3959x.91 – 15 has a myriad of effects depending on the manner in which it is applied to a living subject. These effects usually involve the mutation of the subject's body, immense strength, and extreme hostility. It is presumed that the engineers used it for biological warfare or the "upgrading" of an organism by inplementing them with Xenomorphian DNA.