Half-Life Wiki
Half-Life Wiki
This subject is from the Black Mesa Incident era.
Wiki cleanup This article has yet to be cleaned up to a higher standard of quality.
You can help by correcting spelling and grammar, removing factual errors, rewriting sections to ensure they are clear and concise, and moving some elements when appropriate. Visit our Cleanup Project for more details and, please, notify the administrators before removing this template.
Wiki spoiler This article contains spoilers. Read at your own risk!
Imagecat This article is about the borderworld. For the Half-Life chapter, see Xen (chapter).

"The borderworld, Xen, is in our control, for the time being... thanks to you."
The G-Man[src]

Xen (pronounced "zen" [3]) is the Borderworld, a plane of existence connecting two or more universes (which Xen can be referred to as). 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 also the final retreat of Nihilanth's species and the place where most of the alien species appearing in the Half-Life series migrated to after the Combine had taken over their worlds, and before coming to Earth during the Resonance Cascade.


Xen islands 1

The first island to be seen by Freeman.

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 somewhat 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 atmosphere breathable for humans 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.

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

Xen bacteria things

Massive microorganism-like structures (theorised to be universes) can be seen in the distance.

The exact fate of Xen after the death of the Nihilanth in Half-Life is unknown. 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, a destroyed tank, and a downed fighter jet can be seen, implying that American military forces have failed at their mission.


The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. Although residents of Xen at the time of the Black Mesa Incident, no living organisms are native to Xen, so all species present must have originated somewhere else.[4][5] Water tends to be reddish or green in color.

Gonarch first

The Gonarch is one of the largest creatures to be found on Xen.

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 meters high, four-legged invertebrate known as the Gonarch, being somehow connected with Headcrabs.

Xen first teleport1

An example of the organic-looking asteroid cavern-like areas explored on Xen.

Various harmless creatures also exist in Xen. 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). A large floating harmless amoeba-like organism which "sings" like a bird may be seen all throughout Xen. 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, Gargantuas, 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.

Boids Xen

Boids glide gracefully through the skies of Xen.

Xen trees sun

These tree-like organisms can be found throughout Xen.

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 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[]

After the Combine took over the Xen residents' home worlds, the fleeing aliens retreated to Xen via teleportation and colonized Xen with their architecture and technology (bringing with them invasive wildlife 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, perhaps their hands, due to them being an intelligent species.

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 board at Valve's headquarters seen in early 2004. It is unknown if it is related to Xen, or another border world 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".


  • 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 intelligence", referencing worlds similar to Xen.
  • 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.
    • Additionally, Barney Calhoun and Adrian Shephard have no trouble surviving in Xen even without the HEV Suit Gordon Freeman wears.
    • There is also a living scientist in the Nihilanth "side" chambers, he can be found sitting on a small ledge in the room with the Gargantua.
    • The oxygen monitor in Gordon's suit works as on Earth when he jumps into pools of water on Xen.


Concept art[]

On Xen[]

Xen creatures[]

Cut (RACE-X)[]

Half life 1 Retail[]

Half-Life 2[]

Half-Life: Alyx[]

List of appearances[]


Imagecat The Half-Life Wiki has more images related to Xen.