Xen

"The borderworld, Xen, is in our control, for the time being... thanks to you."

- G-Man

Xen (pronounced "Zen") is the place of origin of many alien species that appear in the Half-Life series.

Introduction


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.

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 sufficiently strong pull to retain the breathable atmosphere encountered there. 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 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. 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.

Ecosystem
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 their enemies. 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 6 meter high, four-legged invertebrate known as the Gonarch, which appears to be the final stage of a Headcrab's lifecycle.



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. 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 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 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 bioluminescent 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.

Criticism
While much of Half-Life takes place in and around the Black Mesa Research Facility, the final few chapters take place in Xen. Some players have complained about this, insisting that the Black Mesa portions are better designed. They praise the jarring effect of "normal" surroundings being invaded by unfamiliar creatures, and claim that this is lost when encountering alien creatures in their own environment.

However, other players have praised the Xen sections because of the imagination and quality of design in its execution. While most games that feature aliens draw inspiration from sources such as Aliens (e.g. Duke Nukem 3D) or generic horror films (e.g. Doom), the aliens and environments of Half-Life have been more carefully crafted with considerable attention to detail (e.g. Xen's ecology; homologous anatomy).

Another source of player complaints is the disproportionately large number of jumping puzzles in Xen. Its unusual physics mean that there are many sections in Xen where the player must perform a series of precisely-executed jumps, with mis-steps punished by death.

Trivia

 * When the Nihilanth teleports the player to the third chamber, there is a hidden scientist on an above platform.
 * 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". This would imply that every dimension has a borderworld, with Xen being Earth's.
 * The smaller ray-like flying creatures are referred to as "boids", probably in reference to Boids, an artificial life simulation of flocking behaviour in birds.