Xen is a border world between the half-life universe, And The Combine's home universe, And Race X's Home universe, And The Nihilanth's home universe, And many other unknown universes, Xen can only be entered through quantum teleportation, It is advised not to fall from a floating island in Xen, Humanity's company named Black Mesa accidentally found Xen in the year 2001, which then caused a resonance cascade which leads to the plot of Half-Life 1