Race X

Race X is a hostile alien race featured in the computer game series Half-Life by Valve Software. They appear only in the Opposing Force expansion pack, and very little is revealed about them. Race X aliens appear late in the timeline of Half-Life, only a few hours before Gordon Freeman has teleported to Xen (an event that takes place in both Half-Life and Opposing Force). No explanation is given for the appearance of Race X aliens on Earth in-game, though the official strategy guide states that the race's intentions are to invade the planet to acquire its natural resources.

Pit Drone
Pit Drones are the first type of Race X aliens encountered in the game. They are short bipedal creatures with yellowish-tan skin and a small head which sports rows of sharp spines.

A Pit Drone's arms end in long scythe-like claws, which it can use to make vicious attacks. Fast and fairly intelligent, they tend to appear in groups of five or six, and are capable of making flanking movements and quickly surrounding their prey. They have two forms of attack: firing their head-spines when at long range, or charging towards the target to deliver a deadly slash with their claws. Some Pit Drones appear not to be capable of the head spine attack and, on closer inspection, have no spines in their head.

Shock Trooper
Shock Troopers are the most intelligent of the Race X aliens. Physically, they are humanoid with four arms and have a posture that makes them vaguely resemble Vortigaunts. Their bodies are covered in plate-like sections, similar in appearance to insect exoskeletons. Their skin is pale blue, and is not mottled or patchy like the skin of Xen creatures. They have a single large eye, whereas Vortigaunts have six. This eye is apparently part of a common morphology between Race X creatures—every Race X alien with a large eye (including the Pit Worm) has two eyelids which regularly blink horizontally across the eyeball and what appears to be "tentacles" surrounding their mouth (which is under the head).

Their primary weapon is the Shock Roach; a living creature, similar to the Alien Grunts' hive-hand. The Shock Roach fires bolts of electricity, which it constantly recharges on its own. The Shock Roach cannot survive without a living host for very long; if its Shock Trooper dies, it will detach itself and pursue a new host. If it nears a player that already has a Shock Roach, it will attack in a similar way to a headcrab before dying.

Shock Troopers also have the ability to regurgitate and throw spore grenades, which they tend to use against distant or concealed enemies. Infant Shock Troopers will forcibly launch the same type of spore when squeezed, and can thus be used as a Spore Launcher weapon. (The Spore Launcher is listed as "baby strooper" in the game's files.) In one of the Spore Launcher's idle animations in Opposing Force, it emits an amphibious purr.

Baby Shock Troopers are effectively tadpoles, an earlier stage that is only a head and torso, with tentacles trailing behind. Beyond the small size and lack of limbs, Baby Shock Troopers are also notable for having gills and two small eyes instead of the adults' single large one. They are amphibious, although immobile outside of water. Baby Shocktroopers are first found kept in a tank in the Sector E Biodome Complex of Black Mesa.

Shock Troopers are usually encountered in small groups, and often fight together with other Race X aliens, whom they seem to be allied with. Shock Troopers also possess a verbal form of communication, uttering strange electronic-sounding noises to each other. A skilled player can learn to recognize certain "phrases" muttered by a Shock Trooper prior to an attack and react accordingly.

Shock Troopers have mastered teleportation technology and use it to travel in and out of combat. They also control the teleportation of Pit Drones and Voltigore. It is interesting to note that at one point, the player observes a Shock Trooper grab a Black Mesa scientist prior to teleporting away; effectively kidnapping him.

Voltigore
The Voltigore, not to be confused with Vortigaunts, are gigantic and lumbering creatures serving as the alien race's heavy support infantry. Voltigore have four legs and two arms that end in scythe-like claws, and are distinguished by vivid purple stripes on their bloated carapace similar to those on a houndeye, though in a different color. These aggressive creatures use their innate ability to harness electrical charges to create a devastating ranged energy bolt attack, bearing some similarity to the Vortigaunt's green energy stream, although the Voltigore's energy attack is purple, and is much more lethal. At close range, they attack viciously with their claws. A player can recognize a Voltigore from afar by the unusual, electric-sounding bellows of the creature. When a Voltigore dies, its carapace explodes, causing massive damage to nearby creatures.

Baby Voltigore can be found later in the game. They are much smaller and weaker, lighter in color, and lack the energy attack. Baby Voltigore are only found in concealed underground nests and are closely guarded by the adults.

Pit Worm
Encountered in a sewage processing area in the lower levels of Black Mesa, this dangerous creature resembles an enormous caterpillar, and bears resemblances to the Gohma boss from the Zelda series. It makes bird-like vocalizations. Its segmented body supports a head dominated by a single large eye. This eye is the pit worm's primary weapon, capable of firing a searing energy beam at nearby enemies. If enemies get too close, the worm also has large claws to strike out with. If the worm is shot in the eye, it will recoil, shielding it with its claws for a few seconds before striking aggressively at the player.

The pit worm obstacle is similar to the tentacle sequence from Half-Life, in that the player cannot simply use his weapons to defeat the creature, but must use the surrounding environment. The process of destroying the pit worm requires the player to dissolve it by turning a valve and activating a gearbox, dumping toxic waste onto it—obvious references to Valve Software and Gearbox Software, the developers of Half-Life: Opposing Force.

Gene Worm
The Gene Worm is the final boss of Half-Life: Opposing Force, and the most mysterious of the Race X aliens. This life form is the equivalent of a "biological resource management factory." It's capable of taking any of Earth's natural resources and assimilating it to make them useful to the aliens' needs. If one fully entered Earth and became productive, it would effectively alter the planet enough to become useful as a new homeworld for Race X.

Physically, the creature resembles a green, worm-like creature with many tentacles, with a beak-like protrusion similar in both shape and position to that of the Xen tentacle. The Gene Worm attacks with its tentacles and also ejects a deadly biotoxin stream from its mouth. Its outer hide is impervious to damage; in order to kill it, the player must blind the creature by shooting both its eyes with the mounted laser guns on either side of the worm. This causes it to summon reinforcements by opening its abdominal cavity, revealing a portal orb that a Shock Trooper will teleport through. The player can inflict serious internal damage by firing at the orb.

The Gene Worm is found deep underground in a seemingly abandoned industrial area of Black Mesa, and appears through a portal opening in a room there, apparently either unwilling or unable to fully enter this world. The room itself has been fitted with mounted weapons which are powered by Xen energy crystals. The player character, Adrian Shephard, is sent down to the portal by a Black Mesa security guard, who remarks "Some of your buddies went down earlier, and I haven't seen 'em since." These soldiers were only able to manage to set up the mounted weapons before their unseen demise, although nothing is seen of their remains.

Sprite
Taking on the appearance of what looks like a flying ball of light, Sprites are creatures that can be seen floating through the facility several times during the game, becoming more and more frequent in later stages of the incident. However, they cannot be harmed and they do not hurt the player in any way. On occasions they even seem to be guiding Adrian. They eventually all lead to the portal that brings the Gene Worm in, possibly acting as energy conduits to construct and charge the gate that connects the two worlds. It is unknown if these are biological aliens or technological creations of Race X.