A powerful, massive entity in control of the armies of Xen, the Nihilanth is the controlling intelligence behind the Xenian invasion of Earth. 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.
The Nihilanth is the controlling intelligence driving the invasion of Earth in the original Half-Life. Dr. 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. 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. The resonance cascade tore open a rift between Xen and Earth, which the Nihilanth exploited through its own powers over space-time to open hundreds of portals throughout the Black Mesa facility. This allowed the Nihilanth's Xenian armies to pour through, and the only possible way Black Mesa's surviving scientists could determine to stop the invasion was to kill the Nihilanth itself.
Gordon Freeman succeeds, and while Earth was saved from the initial threat of Xen, the death of the Nihilanth caused wild and unpredictable portal storms -energetic shockwaves emanating from unstable dimensional tears- to ravage the surface of the planet. These portal storms devastated Human society, depositing many species of Xenian fauna on Earth, like headcrabs and antlions, and forcing humanity to retreat to its cities for protection. Worse yet, the raging portal storms alerted the trans-dimensional armies of the Combine Empire to Earth's location in the dimensional fabric.