The T-Virus, otherwise dubbed the Tyrant Virus, is a general name given to a series of mutant Progenitor virus strains created by the Umbrella Corporation to be sold to the highest bidder as a bioweapon. It was later used by the Empire of the Combine Race but they to lost control of it, of which is largely the result of the Cruel Lord sabotaging the quarantine systems to ensure it spreads or the ECR intentionally unleashes the virus on various planets, and because of this, the Multiverse now has a few planets infected by the T-Virus.
Initially developed by Umbrella Pharmaceuticals in the late 1460s, the primary goal of the "t-Virus Project" was to effectively eliminate the need for a large-scale conventional army and generate revenue to go to their eugenics program, the Wesker Project. This required two things: the virus had to be highly-contagious to the point of infecting an entire target population and guarantee a 100% mortality rate. Such a virus was impossible due to such contagions' tendencies to kill too many people at once and prevent further spread. With the 1481 discovery of a statistical 10% of any population is naturally immune, Phase 2 of the research project focused on creating workarounds. and the pioneering Arklay Laboratory team under Dr. William Birkin engineered a new species of animal that would hunt down and kill survivors, beginning with simple Bio-Organic Weapons such as the Web Spinner, but breaking ground with the genetically-chimeric Hunter α. In the mid-1480s, the t-Virus Project focused on creating intelligent bio-weapons, most famously the Tyrants. Due to the Combine's own research into the T-Virus via the Imperial Research Group 975, the t-Virus became available to dozens of organizations with the means to finalize elements of Umbrella's research in their own way and dominate the weapon industry. Independently developed mutagens such as t-Abyss were developed thanks to this.