For other uses, see Xen (disambiguation).
Xen (pronounced "zen"), also referred to as the Borderworld, is a separate overlapping dimension notably discovered by Black Mesa scientists, who regularly traveled there via portals to collect samples and study the myriad species of flora and fauna within. Xen is described as a "dimensional transit bottleneck--an area of continual contention." It is the final retreat of Nihilanth's race, and a place to which most of the alien species appearing in the Half-Life series migrated to before coming to Earth during the Resonance Cascade.
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 islands floating within a nebular void.
Xen is notable for having greatly reduced gravity compared to Earth's. However, given that Xen is comprised of low mass asteroids, it is surprising that there is any appreciable gravitational pull at all, let alone a pull strong enough to retain an atmosphere. Another feature is that gravity is always directed downwards, rather than towards the asteroid, so that one can actually fall off an asteroid into the void below. While it is possible that the "islands" are small chunks orbiting a larger and unseen planetoid, these unusual physics strongly suggest that Xen occupies an alternate dimension where conventional physics may simply not fully apply.
Xen's atmosphere seems to be similar to that of Earth's, as the Xen wildlife appears to have no trouble surviving on the planet. Furthermore, several human characters including Gordon Freeman and Barney Calhoun are shown to be able to breathe in Xen without any additional life support.
The exact fate of Xen after the death of the Nihilanth in Half-Life is unknown, though it is sometimes mentioned in further installations of the series. In Half-Life 2, Resistance teleports 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 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, showing the American military forces possibly having participation in capturing Xen, as well as the failure at their mission. However, this sequence could quite easily be a mistake and is supposed to take place on Earth, given that the ground texture is Earth sand, the scene would aesthetically fit on Earth at night, and that because the Gold Source Engine can't switch skyboxes mid-level, the skybox would remain that of Xen, instead of Earth.
The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. It has been stated that no species is native to Xen, meaning that, although resident in Xen at the time of Half-Life, all of these species originated elsewhere and were moved there, willingly or unwillingly. While mostly seen as lifeless, the presence of water (reddish-green in color) indicates that the dimension is somewhat 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 - the final stage of a Headcrab's life cycle.
Seen frequently in Xen, and occasionally in Black Mesa, where they transport Alien Grunts, Alien Crafts 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. In the closing sequence of Half-Life, and in several areas of Blue Shift, Boids can be seen flying in flocks, emitting resonant calls.
Other more intelligent species, such as the Alien Controllers and Vortigaunts, form part of an advanced alien civilization with other species, such as the Alien Grunts (made there in the mines) 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 seemingly vestigial third limb in the center of their thorax and vertically-opening mouths, emphasizing the close relation between these species.
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. Fungus-like organisms are very common on Xen. 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, resembling underdeveloped Tentacles. These "trees", often accompanied by hair-like plants and previously mentioned Xen Plant Lights, 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, known as the Bio Sphincters, seen also in Black Mesa Research Facility, that release a blast of air when stepped on. A similar kind was seen on Xen where it opens and closes, and the said opening releases a gust of air like a vent.
 Colonization and civilization
Since the Combine's takeover of would-be Xen creature's former world, they retreated to Xen via teleportation and colonized it 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 Vortigaunts' buildings look like small hills carved by an unknown tool. Various lights, elevators, conveyor belts, pistons, force fields and portals are present in the inhabited parts of Xen, especially the Mines.
Some unidentified Xen machinery made of a moving cylinder held by four legs can be seen near Vortigaunts' shacks; the cylinder pumps in a piston-like motion into the ground, creating green rings that fly off into the sky - this same effect is seen in other locations on Xen, such as along the "chimneys" of the very first island. These machines are apparently managed by the Alien Slaves, as two are briefly seen mimicking the machinery's oscillations with their hands. Two examples can be seen in a functioning state outside, while two others can be seen dismantled in the Mines.
The Alien Grunts are manufactured artificially and can be seen stored in a barrel-like containers. The supreme leader - the Nihilanth - has its own personal chamber which is heavily defended by Vortigaunts, Alien controllers and includes Xen crystals used for regaining strength.
 Behind the scenes
Despite being pronounced as "zen", 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".
According to Marc Laidlaw, Xen was a last-minute addition and the developers didn't know what kind of boss they would have nor what would happen until very late, when they were out of time and had to create an ending. It was known, however, for most of the time during the development, that the player would teleport to an alien world. In the early, on-paper designs of Xen, he was to travel through a giant living organism. However, such an environment proved to be impossible to create at that time.
Marc Laidlaw also said that the big weakness of Xen was that the developers "barely set it up from the beginning," so when the player got there, "there was no sense of inevitability or of resolution/closure. It all felt arbitrary. It was less important to know how it was going to end than to know where it was going to end."
Brett Johnson stated on his website that "the alien world of Xen represented a substantial challenge in both design and execution. While the play may have lacked support, there was some success in the ability to pull off a style that no one had seen before. While building such structures today in a proper 3D package may be simple, it was very difficult with the CSG technology at the time. The tools, technology, and system requirements all had a significant influence on the style of Xen."
Xen appears in the concept art for Episode Three.
 Half-Life story arc
 Concept art
View of Xen, with different creatures, notably the cut Stukabat.
The Alien Craft.
The Alien Controller.
The Alien Grunt.
The Hivehand's Hornet.
The Plant Light.
The Fast Walker.
The Flocking Floater.
The Mr. Friendly.
The Panther Eye.
The Sand Barnacle.
The Healing Chamber.
 Half-Life 2 story arc
 Concept art
 List of appearances
- Half-Life (First appearance)
- Half-Life: Opposing Force
- Half-Life: Blue Shift
- Half-Life: Decay
- Half-Life 2 (Mentioned only)
- Half-Life 2: Raising the Bar
- ↑ Half-Life section on Brett Johnson's personal website
- ↑ Behind Valve Software's Closed Doors on HalfLife.org (archived)
- ↑ Half-Life section on Karen Laur's personal website
- ↑ Half-Life page on Sierra Studios' official website (archived)
- ↑ Half-Life - Lambda Core
- ↑ 6.0 6.1 6.2 6.3 Marc Laidlaw on Xen
- ↑ 7.0 7.1 Marc Laidlaw on the Alien Controllers
- ↑ Half-Life 2: Raising the Bar
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|Half-Life: Opposing Force story arc journey||Succeeded by|
Section A-17 Prototype Labs
|Half-Life: Blue Shift story arc journey||Succeeded by|
Section A-17 Prototype Labs
|Half-Life: Decay story arc journey||Succeeded by|
Sector E Materials Transport
|Retail||Aperture Science · Black Mesa · Black Ops · Combine · Hazardous Environment Combat Unit · Race X · Resistance · Xen|
|Cut||Central Intelligence Agency · Agents|