Race X is a mysterious collective of alien races believed to be from an alternate dimension. Their first (and only) encounter with the human race was during the late stages of the Black Mesa incident, when they followed the Vortigaunts and other Xen denizens through the resonance cascade to Earth. While their motivations, origins and generally anything about them remain unknown, their objective appeared to be the terraformation of Earth through the use of a creature known as the gene worm. However, their invasion was halted by the human soldier, Corporal Adrian Shepherd, who killed the gene worm and sealed the Race X portal in Black Mesa. Since the Black Mesa incident, Race X is not known to have made any further encounter with Humans or Xen species. Whether the Combine Union have ever encountered Race X, or if they are even aware of them, remains unknown.