HL1 article
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.
Wiki spoiler
This article contains spoilers. Read at your own risk!
It might spoil your gaming experience when first playing the related games.
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 dimensions (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 the final retreat of Nihilanth's race and a place where most of the alien species appearing in the Half-Life series migrated after Combine had took over their world 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 first teleport1

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

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 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.

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 resident in Xen at the time of Half-Life, no living organisms are native to Xen, so all species present must have originated from somewhere else.[4][5] While mostly seen as lifeless, the presence of water (reddish-green in color) indicates that the dimension is capable of supporting life.

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.

Boids Xen

Boids glide gracefully through the skies of 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). 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.

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' former world, the fleeing aliens retreated to Xen via teleportation and colonized Xen with their buildings and technology (also 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."
  • 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 has no trouble surviving in Xen even without the HEV Suit Gordon wears or the gas mask worn by Adrian.
    • 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.
  • 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 universe.
  • There is a series of Garry's Mod Fan maps known as "DIM-LOC-2532" which solely is based after the Nihilanth's death, playing around with the theory of the Combine occupying Xen (which likely happened in canon).


Concept art

On Xen

Xen creatures



Half-Life 2

List of appearances


The Half-Life Wiki has more images related to Xen.
Preceded by
Sector F Lambda Complex
Half-Life story arc journey Succeeded by
Preceded by
Sector F Lambda Complex
Half-Life: Opposing Force story arc journey Succeeded by
Sector E Biodome Complex
Preceded by
Section A-17 Prototype Labs
Half-Life: Blue Shift story arc journey Succeeded by
Section A-17 Prototype Labs
Preceded by
Gamma Labs
Half-Life: Decay story arc journey Succeeded by
Sector E Materials Transport
Community content is available under CC-BY-SA unless otherwise noted.