Read more about Xen at: Wikipedia

Official Site: Valve

The Half-Life video game series features many locations set in a dystopia future stemming from the events of the first game, Half-Life. These locations are used and referred to throughout the series. The locations, for the most part, are designed and modeled from real-world equivalent locations in Eastern Europe, but also include science fiction settings including the Black Mesa Research Facility, a labyrinthine subterranean research complex, and Xen, an alien dimension.

The Black Mesa Research Facility (shortened to B.M.R.F) is the primary setting for Half-Life and its three expansions: Opposing Force, Blue Shift, and Decay. The base is a decommissioned ICBM launch complex at an undisclosed New Mexico desert location, which has been converted into a scientific research facility and bears a number of similarities to Los Alamos National Laboratory, Sandia National Laboratories and Area 51. This facility is depicted as a vast series of underground research laboratories as well as surface constructions such as offices, chemical waste disposal plants, and personnel dormitories (even cafeterias, where Gordon Freeman can destroy Doctor Magnusson's microwave casserole), all powered by a hydroelectric dam and connected by an advanced tram system.

Over the course of the series, Black Mesa is revealed to be conducting top-secret research into various fields, such as teleportation and experimental weapons research. Prior to the beginning of Half-Life, scientists experimenting on teleportation discover Xen, a border dimension somehow intrinsically involved in the teleportation process. Creatures and crystals from Xen are subsequently brought back to the facility for testing. At the beginning of Half-Life, one such crystal, revealed in Half-Life 2: Episode Two to have been provided by the G-Man, is put through an anti-mass spectrometer and causes a resonance cascade, tearing the spacetime continuum. As a result, Xen creatures are teleported into the facility and prey on its human inhabitants. There are many notable creatures that come from Xen, such as the Vortigaunt and Headcrab.

The resulting crisis is seen from several points of view in Half-Life and its expansions. In Half-Life, protagonist Gordon Freeman is introduced to the facility in a notable sequence involving very little interactivity. This serves to foreshadow many of the challenges the player will face, as well as the labyrinth-like structure of the game. Eventually, the player fights through the facility and teleports to Xen to try to seal the tear from the other end, where a Xen creature is keeping it open. Blue Shift shows the events from the viewpoint of a security guard, Barney Calhoun, who joins a group of scientists who use the teleportation technology to evacuate survivors from the base. In Decay, another group of scientists attempt to close the tear through their own equipment, after calling in the U.S. Military to assist with the situation. The military situation is shown through the eyes of Adrian Shephard in Opposing Force, where U.S. Marines (referred to as HECU, or Hazardous Environment Combat Unit) are ordered to cover up the incident by killing the entire population of Black Mesa as well as the alien attackers, but are overwhelmed and forced to withdraw, allowing for black operation units to detonate a nuclear warhead in the facility, ultimately destroying it. However, the fracture in the spacetime continuum remains, allowing the Combine to invade and occupy Earth.

Xen has not been a contender in any CBUB matches.