Xen

"The borderworld, Xen, is in our control, for the time being... thanks to you."

- The G-Man

Xen (pronounced "zen" ), or the Borderworld, is a plane of existence connecting two or more dimensions. It was discovered by Black Mesa research scientists, who regularly traveled there via teleportation to collect samples and study the myriad species of flora and fauna within, despite the extremely high risk of doing so.

It is the final retreat of Nihilanth's race, and a place where most of the alien species appearing in the Half-Life series migrated to before coming to Earth during the Resonance Cascade.

Overview
Rather than being a planet or an artificial structure such as a space station, all of the locations of Xen visited by the player during the course of the Half-Life series are on the surfaces of (or within) asteroid-like "islands" floating within a nebular void.



Xen is notable for having greatly reduced gravity compared to Earth's; however, given that Xen is composed of low mass asteroids, it is surprising that there is any appreciable gravitational pull at all, let alone a pull strong enough to retain the breathable atmosphere encountered there. It is possible that, given that Xen resides in another dimension or universe, the entire void contains a breathable atmosphere.

Another feature is that gravity is always directed downwards, rather than towards the asteroids, so that one can actually fall off an asteroid into the void below. It is possible that the "islands" are small chunks orbiting a larger and unseen planetoid and remain in orbit not only due to their orbital velocity but also due to unseen force field (possibly magnetic).

Light behaves oddly in Xen. Looking at the different skybox textures, there are multiple sources for light in the borderworld, either through glowing butts of gas or (in some cases) what seems to be a star or some other stellar body.

The exact fate of Xen after the death of the Nihilanth in Half-Life is unknown, though there is a strong possibility that it remains intact. In Half-Life 2, Resistance teleporters are dependent on Xen for intra-dimensional transportation, since the matter stream utilizes the Borderworld as a slingshot to teleport into local space. The G-Man also stated at the end of Half-Life that he and his "employers" are in control of Xen after the defeat of the Nihilanth. While the G-Man says this, dead Marines (the same ones sent to Black Mesa), a destroyed tank, and a downed fighter jet can be seen, implying that American military forces may have participated in capturing Xen, but mostly implies that they've failed at their mission.

Ecosystem
The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. Although resident in Xen at the time of Half-Life, all of these species originated elsewhere and were forced to flee there to escape attack from their enemies. While mostly seen as lifeless, the presence of water (reddish-green in color) indicates that the dimension is capable of supporting life.



Some species, such as Barnacles, Bullsquid, Headcrabs, and Houndeyes appear to occupy niches similar to various carnivorous terrestrial animals (albeit with unusual anatomical or physiological features). However, even these creatures are dwarfed by the towering six meter high, four-legged invertebrate known as the Gonarch, which appears to be the final stage of a Headcrab's life cycle.



Seen frequently in Xen (and occasionally in Black Mesa, where they transport Alien Grunts), Alien Aircraft glide seemingly without effort through the void. It is uncertain whether these ray-like beings are living creatures or examples of war machines built around advanced biotechnology (such as those used by the Combine). In the closing sequence of Half-Life, and in several areas of Blue Shift, Boids can be seen flying in flocks, emitting resonant calls.

The intelligent species of Xen, such as the Alien Controllers and Vortigaunts, form part of an advanced alien civilization, with species like the Alien Grunts and Gargantua, composing their military. As the player progresses through Half-Life, it becomes clear that this civilization is controlled by a central intelligence, the Nihilanth. Although differing in many ways, the Controllers, Vortigaunts, Grunts, and the Nihilanth share a common morphology that includes a vestigial third limb in the middle of the thorax, suggesting that they are all closely related.



A few creatures that appear in Xen are difficult to categorize as either fauna or flora due to their unusual appearance. Stationary bio-luminescent stalks appear in large numbers throughout Xen and will retract when approached. The much larger and extremely aggressive (but rarely encountered) Tentacles pose a major hazard to those who wander near their pits. There are also bizarre tree-like structures dotted around the landscape, which may be juvenile or even a separate species of tentacle. The "trees" sway gently and appear harmless when observed from a distance, but stab viciously at any creature that enters their immediate vicinity. There are also strange trampoline-like plants seen in Black Mesa and Xen that release a blast of air when stepped on. A similar kind was seen on Xen which opens and closes, releasing a gust of air like a vent.

Colonization and civilization
Since the Combine's takeover of the Xen residents' former world, they retreated to Xen via teleportation and colonized Xen with their buildings and technology (also bringing with them invasive wild-life and pests). The buildings for the Vortigaunts were primitive and lacked luxury or necessary needs of survival (besides the healing pool in the back) due to their social class as slaves. The Vortigaunt's buildings look like small hills carved by an unknown tool.

The military castes live in an industrialized, floral Xen Base. The Nihilanth has its own personal chamber which is heavily defended by Vortigaunts, and Alien Controllers.

Behind the scenes

 * The "zen" pronunciation of the name is given in Half-Life. First by the "shotgun scientist" met in Level A of the Lambda Reactor Core in the Lambda Complex, then by the G-Man at the end of the game, and by Alyx Vance in Half Life 2 chapter Entanglement.


 * "Xen 2" is mentioned as an item cut from Half-Life 2 on a board at Valve's headquarters seen in early 2004. It is unknown if it is related to Xen, or another borderworld like Xen, or a direct or indirect reference to Xen.


 * The term "Xen" is seemingly related to the Greek term "Xenos" (ξένος), translated most of the time as "foreigner", "stranger", or "alien". "Xenophobia" is the most common term stemming from it. Furthermore, the WAD file containing the Xen textures (and most of the textures themselves) in the Half-Life game files is named "xeno.wad".

Trivia

 * Near the chapter "DARK ENERGY", in the chapter "OUR BENEFACTORS", in the carrier sequence, near the last upwards (with the first view of the full City 17) scene, you can see an unknown alien never seen on the other episodes or even Half-Life 1. That model is only used that time.
 * In a brief sequence near the end of Half-Life 2, Doctor Breen can be overheard describing "worlds stretched thin across the membrane where dimensions intersect" or "Gas giants inhabited by vast meteorological intelligences." It is likely that Xen is one of the worlds Breen describes here, or even the "membrane".
 * It seems that Xen's atmosphere is not dissimilar to that of Earth's, as the Xen wildlife appears to have no trouble surviving on Earth.
 * There is in fact one living scientist in the Nihilanth "side" chambers, he can be found sitting on a small ledge in the room with the Gargantua.
 * It is a common misconception that many of the creatures seen in the Half-Life series originate from Xen. This is actually untrue, as all of the Nihilanth's race retreated there in the stead of a Combine invasion on their own planet.
 * In the Doom mod Massmouth (as well as its sequel Massmouth 2), there is a chart with a list of names of people and planets they were sent to. One of the rows says "Freeman" and the planet "Xen".

List of appearances

 * Half-Life
 * Half-Life: Opposing Force
 * Half-Life: Blue Shift
 * Half-Life: Decay
 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Source Particle Benchmark
 * Portal
 * Half-Life 2: Episode Two