{{Location Infobox
|image=FileXen islands 1.jpg|250px
|era=Black Mesa Incident
|maps=*c4a1 to c5a1 (HL - ''Xen'' to ''Endgame'')
*of3a2 to of6a5 (Op4 - ''We Are Not Alone'' to ''Worlds Collide'')
*of7a0 (Op4 - ''Conclusion'')
*ba_xen1 to ba_xen6 (BS - ''Focal Point'')
*ba_outro (BS - ''Deliverance'')
*ht99alien (HLD - ''Xen Attacks'')
|designer=*Brett Johnsonhttp//www.brett3d.com/ld_fps.html ''Half-Life'' section on Brett Johnson's personal website (level design)
*Greg Coomer{{IA|19991105192606/http//www.halflife.org/articles/article2-page2.html|Behind Valve Software's Closed Doors|HalfLife.org}} (level design)
*Karen Laurhttp//www.norsecode.com/klaur/hl/default.htm ''Half-Life'' section on Karen Laur's personal website (textures)
*Yatsze Mark{{IA|19990220144204/http//www.sierrastudios.com/games/half-life/archives.html|''Half-Life'' page|Sierra Studios' official website}} (textures)

{{Quote|The borderworld, Xen, is in our control for the time being, thanks to you. Quite a nasty piece of work you managed over there. I am impressed.|The G-Man|Half-Life|Filegman nasty.ogg}}

'''Xen''' (pronounced "zen"''Half-Life'' - Lambda Core), also referred to as the '''Borderworld''', is a separate overlapping dimension notably discovered by Black Mesa scientists, who regularly traveled there via Teleportation|portals to collect samples and study the myriad species of flora and fauna within. Xen is described as a "dimensional transit bottleneck--an area of continual contention." It is the final retreat of Nihilanth's race, and a place to which most of the alien species appearing in the Half-Life and Portal universe|''Half-Life'' series migrated to before coming to Earth during the Resonance Cascade.

FileXen first island.jpg|thumb|left|200px|The first island to be seen by Freeman.

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) asteroid islands floating within a nebular void.

FileXen first teleport1.jpg|200px|thumb|An example of the highly organic asteroid cavern-like areas explored on Xen.

Xen is notable for having greatly reduced gravity compared to Earth's. 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 an 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 WikipediaAsteroid|planetoid, these unusual physics strongly suggest that Xen occupies an alternate dimension where conventional physics may simply not fully apply.

Xen's atmosphere seems to be similar to that of Earth's, as the Xen wildlife appears to have no trouble surviving on the planet. Furthermore, several human characters including Gordon Freeman and Barney Calhoun are shown to be able to breathe in Xen without any additional life support.

The exact fate of Xen after the death of the Nihilanth in ''Half-Life'' is unknown, though it is sometimes mentioned in further installations of the series. In ''Half-Life 2'', Resistance Teleportation|teleports are dependent on Xen for intra-dimensional transportation, since the matter stream utilizes the Borderworld as a slingshot to teleport into local space. The The G-Man|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 Hazardous Environment Combat Unit|Marines, the same ones sent to Black Mesa Research Facility|Black Mesa, a destroyed tank, and a downed fighter jet can be seen, showing the American military forces possibly having participation in capturing Xen, as well as the failure at their mission. However, this sequence could quite easily be a mistake and is supposed to take place on Earth, given that the ground texture is Earth sand, the scene would aesthetically fit on Earth at night, and that because the Gold Source Engine can't switch skyboxes mid-level, the skybox would remain that of Xen, instead of Earth.
The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. It has been stated that no species is native to Xen,Marc Laidlaw on ReferencesMarc Laidlaw emails#On the Alien Controllers|the Alien Controllers meaning that, although resident in Xen at the time of ''Half-Life'', all of these species originated elsewhere and were moved there, willingly or unwillingly.Marc Laidlaw on ReferencesMarc Laidlaw emails#On the Xen.2FCombine relation|Xen While mostly seen as lifeless, the presence of water (reddish-green in color) indicates that the dimension is somewhat capable of supporting life. 

FileGonarch first.jpg|thumb|left|200px| The Gonarch is one of the largest creatures to be found on Xen.

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 - the final stage of a Headcrab's life cycle.''Half-Life 2 Raising the Bar''

FileBoids Xen.jpg|thumb|right|200px|Boids glide gracefully through the skies of Xen.

Seen frequently in Xen, and occasionally in Black Mesa, where they transport Alien Grunts, Alien Crafts 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. In the closing sequence of ''Half-Life'', and in several areas of ''Blue Shift'', Boids can be seen flying in flocks, emitting resonant calls.

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 (made there in Mines|the mines) 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, emphasizing the close relation between these species.

FileXen trees sun.jpg|thumb|left|200px|These Xen Tree|tree-like organisms can be found throughout Xen.

A few creatures that appear in Xen are difficult to categorize as either WikipediaFauna|fauna or WikipediaFlora|flora due to their unusual appearance. Stationary Xen Plant Light|bio-luminescent stalks appear in large numbers throughout Xen and will retract when approached. Xen Fungus|Fungus-like organisms are very common on Xen. 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, resembling underdeveloped Tentacles. These "trees", often accompanied by Xen Hair|hair-like plants and previously mentioned Xen Plant Lights, 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 Alien Trampoline|trampoline-like plants, known as the Bio Sphincters, seen also in Black Mesa Research Facility, that release a blast of air when stepped on. A similar kind was seen on Xen where it opens and closes, and the said opening releases a gust of air like a vent.

===Colonization and civilization===
Since the Combine's takeover of would-be Xen creature's former world, they retreated to Xen via teleportation and colonized it 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 Vortigaunts' buildings look like small hills carved by an unknown tool. Various lights, elevators, conveyor belts, pistons, force fields and portals are present in the inhabited parts of Xen, especially Mines|the Mines.

Some unidentified Xen machinery made of a moving cylinder held by four legs can be seen near Vortigaunts' shacks; the cylinder pumps in a piston-like motion into the ground, creating green rings that fly off into the sky - this same effect is seen in other locations on Xen, such as along the "chimneys" of the very first island. These machines are apparently managed by the Alien Slaves, as two are briefly seen mimicking the machinery's oscillations with their hands. Two examples can be seen in a functioning state outside, while two others can be seen dismantled in the Mines.

The Alien Grunts are manufactured artificially and can be seen stored in a barrel-like containers. The supreme leader - the Nihilanth - has its own personal chamber which is heavily defended by Vortigaunts, Alien controllers and includes Xen crystals used for regaining strength.

==Behind the scenes==
Despite being pronounced as "zen", the term "Xen" is seemingly related to the Greek term "WikipediaXenos (Greek)|Xenos" (ξένος), translated most of the time as "foreigner", "stranger", or "alien". "WikipediaXenophobia|Xenophobia" is the most common term stemming from it. Furthermore, the WAD file containing the Xen textures (and most of the textures themselves) in the ''Half-Life'' game files is named "xeno.wad".

According to Marc Laidlaw, Xen was a last-minute addition and the developers didn't know what kind of boss they would have nor what would happen until very late, when they were out of time and had to create an ending. It was known, however, for most of the time during the development, that the player would teleport to an alien world. In the early, on-paper designs of Xen, he was to travel through a giant living organism. However, such an environment proved to be impossible to create at that time.ReferencesMarc Laidlaw emails#On the endings of Half-Life and Half-Life 2|Marc Laidlaw on the endings of ''Half-Life'' and ''Half-Life 2''

Marc Laidlaw also said that the big weakness of Xen was that the developers "barely set it up from the beginning," so when the player got there, "there was no sense of inevitability or of resolution/closure. It all felt arbitrary. It was less important to know how it was going to end than to know where it was going to end."

Brett Johnson stated on his http//www.brett3d.com/media/hl_19.html website that "the alien world of Xen represented a substantial challenge in both design and execution. While the play may have lacked support, there was some success in the ability to pull off a style that no one had seen before. While building such structures today in a proper 3D package may be simple, it was very difficult with the CSG technology at the time. The tools, technology, and system requirements all had a significant influence on the style of Xen."

Xen appears in the concept art for ''Future of the Half-Life series|Episode Three''.

===''Half-Life'' story arc===
====Concept art====

FileXen view.jpg|View of Xen, with two cut creatures, the Flocking Floater and the Kingpin.
FileXen vista full.jpg|View of Xen, with different creatures, notably the cut Stukabat.
FileInterloper concept.jpg|The cabal for an unknown map.
FileBigmomma concept.jpg|The cabal for the ''Half-Life'' chapter ''Gonarch's Lair''.


FileXen D1 ordering.jpg|Early screenshot.
FileC1a0e pool.jpg|Bullsquids drinking from a pool.
FileFungus on Xen.jpg|Xen Fungi and a Xen Plant Light, parts of the Xen plant life.
FileXen thumpers.jpg|Unidentified machinery.
FileLeft xen thumper ring.jpg|Vortigaunt controlling one of the two previous devices, while a ring flies off.
FileXen thumpers stored.jpg|Dismantled versions stored in the mines.
FileAlien grunt factory first.jpg|The mines.
FileTeleport Nihilanth.jpg|The teleport preceding the Nihilanth.
FileOf4a xen displacer1.jpg|Floating islands seen throughout Xen.
FileOf4a xen displacer2.jpg|Ditto, close-up.
FileFocal Point.jpg|Xen as first seen by Barney Calhoun.
FileBa xen20002.jpg|Xen crystals and Xen Trees in a Xen grotto.
FileBa xen20003.jpg|Barnacles in another grotto.
FileBa xen20005.jpg|A Healing Pool with Xen Hair around it.
FileBa focuser03.jpg|Large Xen crystals making up the Xen "focal point."
FileBa xen50001.jpg|Survey team equipment on Xen.


FileManta ray double.jpg|The Alien Craft.
FileAlien Controller2.jpg|The Alien Controller.
FileHl agrunt01.png|The Alien Grunt.
FileHl barnacle.png|The Barnacle.
FileBoid hl.jpg|The Boid.
FileC4a1 bug03.jpg|Bug (Xen)|Bugs.
FileBullsquid hl.png|The Bullsquid.
FileXen fungus model.jpg|The Xen Fungus|Fungus.
FileHl garg.png|The Gargantua.
FileGonarch model.jpg|The Gonarch.
FileXen Hair.png|The Xen Hair|Hair.
FileHl headcrab.png|The Headcrab.
FileHivehand projectile.jpg|The Hivehand's Hornet.
FileHoundeye.jpg|The Houndeye.
FileIchthyosaur hl1.jpg|The Ichthyosaur.
FileLeech HL1.jpg|The Leech.
FileXen Plant Light model.jpg|The Xen Plant Light|Plant Light.
FileHl nihilanth.png|The Nihilanth.
FileProtozoa model.jpg|The Protozoan.
FileSnark w.jpg|The Snark.
FileTentacle.jpg|The Tentacle.
FileXen tree.jpg|The Xen Tree|Tree.
FileVort hl.png|The Vortigaunt.
FileChumtoad model.jpg|The Chumtoad.


FileArcher model.jpg|The Archer.
FileCharger model front.jpg|The Charger (Xen creature)|Charger.
FileFast Walker.jpg|The Fast Walker.
FileFlocking Floater model.jpg|The Flocking Floater.
FileKingpin-model.jpg|The Kingpin.
FileHl friendly.png|The Mr. Friendly.
FileRed Panther Eye model.jpg|The Panther Eye.
FileSand Barnacle model.jpg|The Sand Barnacle.
FileSnapbug model.jpg|The Snapbug.
FileStukabat model.jpg|The Stukabat.
FileSphere open.jpg|The Sphere.
FileUnidentified alien.jpg|Unidentified creature, originally designed for ''Quiver''.


FileC4a1e0001.jpg|The Healing Chamber.
FileXen thumper brush.jpg|Unidentified machinery found on Xen, used by Vortigaunts and emitting green rings.
FileBreather.png|An unused, unknown device, called the Breather.


FileSky alien1.png|alien1.
FileSky alien2.png|alien2, unused.
FileSky alien3.png|alien3, unused.
FileSky neb1.png|neb1, unused.
FileSky neb2b.png|neb2b, unused.
FileSky neb6.png|neb6.
FileSky neb7.png|neb7.
FileSky xen8.png|xen8.
FileSky xen9.png|xen9.
FileSky xen10.png|xen10.

===''Half-Life 2'' story arc===
====Concept art====

FileEp3 xen bubble1.jpg|''Episode Three'' concept art of an unknown entity on Xen.
FileEp3 xen bubble2.jpg|Ditto.
FileEp3 xen thing1.jpg|Ditto.
FileEp3 xen thing2.jpg|Ditto.
FileEp3 xen thing3.jpg|Ditto.
FileEp3 xen thing4.jpg|Ditto.
FileEp3 xen islands.jpg|''Episode Three'' concept art of Xen islands.
FileEp3 xen advisor.jpg

==List of appearances==
*''Half-Life'' {{1st}}
*''Half-Life Opposing Force''
*''Half-Life Blue Shift''
*''Half-Life Decay''
*''Half-Life 2'' {{Mo}}
*''Half-Life 2 Raising the Bar''

{{Imagecat|Xen images}}

|title=''Half-Life'' story arc journey
|before=Sector F
|title=''Half-Life Opposing Force'' story arc journey
|before=Sector F
|after=Sector E
|title=''Half-Life Blue Shift'' story arc journey
|before=Section A-17 Prototype Labs
|after=Section A-17 Prototype Labs
|title=''Half-Life Decay'' story arc journey
|before=Gamma Labs
|after=Sector E Transport|Sector E Materials Transport
CategoryXen locations
CategoryHalf-Life Opposing Force
CategoryHalf-Life Blue Shift