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Race X is the designation given to an alien race which serve as the main antagonists of Half-Life: Opposing Force. They invade the Black Mesa Research Facility late in the Black Mesa Incident.
Race X appears teleport around Black Mesa and have one Xen crystal-powered teleportation device on Xen. They used the Xen teleporter during to bring Sprites prior to the invasion. They were not particularly well adapted to the Borderworld, and never had a notable presence on Xen.
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". The designation "X" was used by Black Mesa to refer to the dimension and its inhabitants.
The aliens of Race X have no relation to those of Xen, and did not cooperate with them. 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. They might also have used the Gene Wormportal, which may have been the original portal to the Race X homeworld in Black Mesa.
The smaller Race X troops begin to make way for the Gene Worm. Their eventual goal were stopped due to Adrian Shephard. It is likely that the rift to their dimension was sealed off by Black Mesa's destruction, removing their only access point to Earth and preventing them from spreading across the planet.
Some of the Race X creatures are used by the Gene Worm is also present in Black Mesa. Despite their physical differences, Race X and Xen aliens do have some things in common. For example, both Race X and Xenian aliens use organic weapons in combat (Race X Shock Troopers using Shock Roaches and Spore Launchers while the Xenian Grunts use the Hivehand) and teleportation technology.