Uroboros (named after the legendary Egyptian symbol of renewal depicting a serpent devouring its own tail) is an insidious mutagenic virus from the Resident Evil games. It is a highly advanced strain of the original Progenitor virus discovered by Umbrella in the 1960's, created in the late 2000's by former Umbrella scientist and bioterrorist, Albert Wesker.
With resources and assistance from the pharmaceutical company TRICELL, Wesker developed Uroboros as his means of creating a new world order. The purpose of the virus was to force the evolution of the human race, to develop a new "master race" of humanity: people that possessed superior intellect and superhuman abilities like those of Wesker himself. Only humans of a particular gene type would reap the benefits of the virus whilst other humans and animals would be killed and/or mutated into horrific creatures. Wesker attempted to infect the entire Earth with Uroboros, knowing that it would wipe out the majority of the human race and enhance the few survivors by making them superhumans like himself, making him the creator - the "god" - and ruler of a new age. He attempted to saturate the planet's atmosphere with the virus, but his insane plan was thwarted by his former S.T.A.R.S. subordinate and hated rival, Chris Redfield.
While Wesker perished, Uroboros did not die with him. His Uroboros Research Facility in Africa remained abandoned and contained stockpiles of the virus and research data that would later be acquired by Alex Wesker, the only other remaining subject of Oswell E. Spencer's Wesker Project, who would use the virus to create bio-weapons of her own and continue the work that Spencer and Albert had been unable to finish: the achievement of immortality.