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Xen is a fictional world that appears in the science fiction video game Half-Life (Valve Software, 1998); the expansion packs Half-Life: Opposing Force (Valve Software and Gearbox Software, 1999); Half-Life: Blue Shift (Valve/Gearbox, 2001); and Half-Life: Decay (Valve/Gearbox, 2001). Xen is sometimes referred to as the border world.


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Rather than being a planet or an artificial structure such as a space station, all of the locations of Xen visited by Gordon Freeman in the course of Half-Life (and by Adrian Shephard in Half-Life: Opposing Force, and Barney Calhoun in Half-Life: Blue Shift) are on the surfaces of, or within, small asteroids (or islands) floating within what appears to be a nebular void. Xen is first introduced to the player by the resonance cascade created by the Black Mesa Incident. The resonance cascade teleports the player temporarily to a swamp-like area of Xen.

Compared to the gravitational pull experienced on Earth, Xen is notable for having somewhat reduced gravity. However, given that Xen comprises low mass asteroids, it is surprising that there is any appreciable gravitational pull at all. Another noticeable 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 not fully apply. This is also hinted at when it is referred to as a "borderworld"[1]. Although there are dead scientists from previous expeditions that can be found wearing helmets in Xen, it is fully breathable, as proven by how Gordon Freeman, Adrian Shepherd and Barney Calhoun lack any kind of equipment that could provide them with breathable air when they go to Xen.

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". The "membrane" here is a reference to brane theory, a branch of string theory.


References in Half-Life 2 and its succeeding episodes by Vortigaunts suggest that there was a history between them and the Combine, previously to that of the resonance cascade. What is suggested is that the aliens found on Xen are not a native species, but retreated there after a battle with the Combine. This can be said about the Vortigaunts and Nihilanth, but it is uncertain where the other Xenian animals and fauna came from.[2]


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The Xen asteroids are home to a wide variety of different plant and animal species, 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 the Combine Empire[3][4] (see also the Combine article).

Some species, including barnacles, bullsquid and houndeyes, appear to occupy niches similar to terrestrial animals, albeit with unusual anatomical or physiological features. While the headcrabs that appear in the Black Mesa Research Facility are mature, juvenile forms of them (informally known as "baby headcrabs") also appear in Xen. These are produced by another Xen resident, the Gonarch, a towering 6 m-high four-legged giant that continually gives birth to infant headcrabs.

A few creatures that appear in Xen are less easy to categorize. Stationary attractive bioluminescent stalks appear in large numbers throughout Xen, which retract when approached, and like several Xen organisms, it is uncertain whether these should be considered plants or animals. There are also bizarre tree-like structures dotted around the landscape, which seem to be more sedate versions of the tentacles. They sway gently and appear harmless when observed from a distance, but stab viciously at any creature that enters their immediate vicinity. These tentacle-trees are frequently accompanied by one of the bioluminescent stalks, but the two organisms are not the same creature, as tentacle-trees and stalks also occur separately throughout Xen. Also, in the second level of the chapter "Interloper", there are some devices which bear a striking resemblance to restrictors from Half-Life 2, devices which are used to repel Antlions.

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These tree-like organisms can be found throughout Xen

Other, more intelligent species, such as the Controllers and Vortigaunts, form part of an alien hierarchy. As the player progresses through Half-Life, it becomes clear that this hierarchy 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 centre of their thorax and vertically-opening mouths.

Seen frequently in Xen and occasionally in Black Mesa, where they transport Alien Grunts, manta ray-like beings are often encountered gliding through the void. These creatures come in two forms; one with a "split" head and a double tail, and the other with a rounder head and a single tail. The beings appear to follow preset patterns of flight, and do not react to attack or contact with other organisms. However, they are capable of emitting beams of energy, the double-tailed type was seen attacking an Osprey aircraft in the opening sequence of Half-Life: Opposing Force, and several times in Half-Life flying mantas zoom past with plane-like shock waves and Alien Grunts dropping from them.

In the closing sequence of Half-Life, and in several areas of Blue Shift, a different kind of aerial creature is observed, emitting soothing resonant sounds and flying rapidly between Xen's islands in a flock. These have no role in gameplay, and just form part of the backdrop of a tour of Xen the G-Man takes Gordon Freeman on. They are referred to as "boids".[5]


  1. The G-Man, end cinematic Half-Life 1
  2. Marc Laidlaw e-mail about Xen, Planet Half-Life Mailbag
  3. Marc Laidlaw e-mail about Xen, Planet Half-Life Mailbag
  4. Marc Laidlaw e-mail about Alien Controllers, forum
  5. The source code model name for this creature is "boid.mdl", and they are referred to as "boids" in subroutines dealing with their behaviour
  • Bell, Joseph (1998). Half-Life: Prima’s Official Strategy Guide. Prima Games. ISBN 0-7615-1360-4. 

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