Their eventual goal were stopped due to Adrian Shephard. The smaller Race X troops begin to make way for the Gene Worm. They might also have used the Gene Wormportal, which may have been the original portal to the Race X homeworld in Black Mesa. The Resonance Cascade probably affected portals linked to their world, allowing them to teleport into the Black Mesa facility in great numbers Some apparently teleported into Xen as well. The aliens of Race X have no relation to those of Xen, and did not cooperate with them. The designation 'X' was used by Black Mesa to refer to the dimension and its inhabitants.
Race X was discovered by chance by Black Mesa scientists when they opened a rift to another dimension than that of Xen during their portal technology experiments, which lay 'Somewhere beyond Xen'. They were not particularly well adapted to the Borderworld, and never had a notable presence on Xen. They used the Xen teleporter during to bring Sprites prior to the invasion. Race X appears teleport around Black Mesa and have one Xen crystal-powered teleportation device on Xen. They invade the Black Mesa Research Facility late in the Black Mesa Incident.
Race X is the designation given to an alien race which serve as the main antagonists of Half-Life: Opposing Force.