Xen (pronounced as "zen" ) is the Borderworld, a plane of existence connecting two or more dimensions. 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 was the final retreat of Nihilanth's race and a place where most of the alien species migrated after Combine had taken over their world and before coming to Golmar Earth during the Resonance Cascade.
Overview[]
Rather than being a planet or an artificial structure such as a space station, it is considered to be nothing more than asteroid-like "islands" floating within a nebular void.
Xen is notable for having somewhat reduced gravity compared to a planet; 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 is unknown. 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 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.
Ecosystem[]
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 the Black Mesa Incident, no living organisms are native to Xen, implying that all species present must have originated from somewhere else. Water tends to be reddish or green in color.
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.
Various harmless animals also exist in 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). A large floating harmless amoeba-like organism which "sings" like a bird may be seen all throughout Xen. 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. It became clear that this civilization was controlled by a central intelligence, the Nihilanth. Although different 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.
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.