Unknown to Gordon until the events of Half-Life 2: Episode Two, the crystal sample that initiated the resonance cascade was provided by the G-Man, apparently taken from the Nihilanth's chamber on Xen itself. Gordon Freeman inadvertently initiates a resonance cascade in the dimensional fabric between Earth's reality and Xen when he places a sample of crystal into the anti-mass-spectrometer of the Anomalous Materials department of the Black Mesa Research Facility. The Nihilanth is the controlling intelligence driving the invasion of Earth in the original Half-Life.

Able to project up to ten powerful balls of energy at the player as well as release a homing teleportation portal, the Nihilanth is the final obstacle Gordon Freeman must overcome to save Earth from the extra-dimensional aliens of Xen. A powerful, massive entity in control of the armies of Xen, the Nihilanth is the controlling intelligence behind the Xenian invasion of Earth.