Xen, Half-Life Wiki

Xen (pronounced "zen" ) is the Borderworld, a plane of existence connecting two or more dimensions (which Xen can be referred to as). 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 is also the final retreat of Nihilanth's race and the place where most of the alien species appearing in the Half-Life series migrated to after

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Xen, Half-Life Wiki

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