Xen exists between different dimensions, hence its designation as a borderworld, and has been visited by alien beings capable of inter-dimensional travel. It has become a refuge for several creatures, including the Vortigaunt, which have fled their home dimensions due to disaster or hostile invasion. Xen had also been visited by humans prior to the Black Mesa Incident; scientists from Black Mesa would regularly teleport to Xen in order to collect flora, fauna and mineral samples for study, despite the extreme risk of falling prey to the numerous hostile creatures inhabiting the region.
In the first Half-Life game, Dr. Gordon Freeman visited Xen in order to try and seal off the breaches between that world and Earth. He encountered Xen's dominant life-form, the Nihilanth, which was commanding many of the creatures attacking Black Mesa. Freeman defeated the Nihilanth and since then Xen has apparently fallen under the control of the G-Man's "employers".
Xen may or may not have any indigenous life-forms since all of the creatures encountered there so far have originated from other worlds. Below is a list of all life-forms thus far encountered there.