Race X is the name given to a mysterious alien race originating from Half-Life: Opposing Force. They featured in City 17 Street as villains in mid-2013 who attempted to launch an invasion on earth.
In the aftermath of their failed invasion, many Race X being were either killed or imprisoned. During the Clandestine Company's planned invasion, they released captured Race X beings in order to join their cause and worldwide syndicate as part of a plan to dominate the multiverse. Race X beings gave their knowledge of teleportation and time travel to the Company, which helped them retroactively manipulate the past and spread their influence.
The Race X assisted another alien race known as "Demons" in order to takeover City 17 and help the Clandestine Agents subdue citizens.
However during the Clandestine Occupation of City Seventeen, the Race X started taking over parts of the city and making them their territories and got into a conflict with the Clandestine Company.
A conflict began brewing on the Race X Homeworld (located in another dimension) in which the Combine began invading. The Elders transported Nathan Johnson briefly to the homeworld to demonstrate that even though there was current "peace" on earth, there were greater goings on in the universe.