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) asteroids ("islands") floating within what appears to be a nebular void. In Half-Life, Opposing Force and Blue Shift, there is water inside these islands. The majority of flowing water is red and greenish brown, however there are pools of blueish water that have a healing effect on the player. The presence of water proves that Xen can support life.
Compared to the gravitational pull experienced on Earth, Xen is notable for having somewhat reduced gravity. 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 the breathable atmosphere encountered there (it could be that if Xen was a dimension of its own, the entire void is a breathable 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 not fully apply. This is also hinted at when Xen is referred to as a "borderworld".
The fate of Xen is unknown, though it is possible that it was more or less destroyed due to the death of the Nihilanth. On the other hand, Resistance teleporters are dependent on Xen for intra-dimensional transportation, since the matter stream utilize the relay device there to "swing around" Xen 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, implying that American military forces may have participated in capturing Xen, but mostly implies that they've failed at their mission.
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), manta ray-like beings glide seemingly without effort 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. These 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 and the double-tailed variant was seen attacking an Osprey aircraft in the opening sequence of Half-Life: Opposing Force. 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, a different kind of aerial creature resembling smaller ray creatures can be observed. These play no role in gameplay, and merely form part of the backdrop of Xen.
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 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, 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 share certain features with animals but also of plants and pose a major hazard to those who wander near their pits. There are also bizarre tree-like structures dotted around the landscape, which seem to be more sedate versions of the Tentacles. 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 blow out air.
Colonization and civilizationEdit
Since the Combine's takeover of would-be Xenian's 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. As for their military class they live in an a high-tech floral industrialized-like Xen Base. As for the supreme leader the Nihilanth has its own personal chamber which is heavily defended by Vortigaunts and alien controllers and Xen crystals to gain strength to destroy would be interlopers.
- Despite being pronounced as "zen", the term "Xen" might be 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.
- 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 is possible that Xen's atmosphere is not dissimilar to that of Earth's, as the Xen wildlife appears to have no trouble surviving on Earth. Gordon's lack of a helmet, and his ability to breath on Xen, offers more evidence to this.
- Half-Life: Opposing Force
- Half-Life: Blue Shift
- Half-Life: Decay
- Half-Life 2
- Half-Life 2: Raising the Bar