Xen

Xen (pronounced "Zen") is the origin of the alien species that appear in the Half-Life series. Xen is sometimes referred to as the border world.

Introduction
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.

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 it is referred to as a "border world".

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". It is likely that Xen is one of the worlds Breen describes here, or even the "membrane where dimensions intersect".

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 (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. They 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.

Other, more intelligent species, such as the Controllers and Vortigaunts, form part of an advanced alien civilization. Some species, such as the Alien Grunts and Gargantuas, appear to form the military arm of this organisation. 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 centre of their thorax and vertically-opening mouths, suggesting that they are closely related species.

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, and the double-tailed type 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 is observed, emitting soothing resonant sounds and flying rapidly between Xen's islands in a flock (see the final screenshot below). These play 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", probably in reference to Boids, an artificial life simulation of flocking behaviour in birds.

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 an entirely alien 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.