Xen

Indoor pools on Xen

Xen is the name given to the dimension of origin of the aliens who came to Earth during the Black Mesa Incident, Xen consists of a series of rocky islands floating in a void. Some of the smaller islands are mobile, while the larger ones, which are all inhabited, don't appear to be. Xen was first discovered by human scientists at Black Mesa, who, during the process of developing relible teleporter technology, discovered that it was linked to the process in some way, a sort of 'Borderworld'. Up until the Resonance Cascade, many samples of Xen lifeforms and were taken back to Earth by the survey teams (wearing HEV suits) for studying and research , while the crystalline growths which grow on Xen were taken to be analyzed. Xen was the home of an advanced civilization, led by a being called the Nihilanth. The civilization was composed of Vortigaunts, who were menial slaves, Alien Controllers, who commanded them, and Alien Grunts, who served as the primary attack and defence force.

None of the aliens are native to Xen, having fled there from the Combine Empire with the help of the Nihilanth