The Nihilanth has the appearance of a gigantic foetus with warped proportions: its head enormous and bulbous, its arms long and gangly (except for the third one extending from its torso) and its legs are shrivelled to the point of uselessness, forcing it to rely on a mechanical levitation device. The Nihilanth's head is held up by a cord extending from the base of the spine to the back of the skull. Further inspection of its hideous body indicates that it had at some point been subjected to some manner of surgical procedure.
Before the events of Half-Life, the Nihilanth ruled over numerous enslaved creatures on its homeworld. However, its homeworld was invaded by the multidimensional empire known as the Combine, which attempted to enslave the Nihilanth and its subjects. The Nihilanth and its minions fled their homeworld, though not without suffering horrific mutilations brought on by Combine experimentation. They fled to the borderworld of Xen, forced to adapt to an unstable ecosystem.
The time would come when an interdimensional rift would be formed between Xen and Earth, caused by a resonance cascade phenomenon brought about accidentally by the scientists at Black Mesa Lambda Labs. Using its own power, the Nihilanth forced the rift to stay open, allowing it to send invasion forces to Earth via portals. Earth possessed a more stable ecosystem than Xen, providing a more suitable environment for the Nihilanth and its forces to live in. All that was required was the removal of its current dominant race.
From its lair on Xen, the Nihilanth maintained the portals to Earth. However, one human scientist - Dr. Gordon Freeman - had heavily armed himself and was proving to be a considerable threat to its invasion parties. Freeman then brought the fight to Xen itself when he passed through the interdimensional rift between the two worlds, invading the Nihilanth's lair. The Nihilanth opened communication with Freeman telepathically, taunting him and threatening him with death, but the human was resolute in his mission to destroy the alien threat. When Freeman reached the Nihilanth's lair, the Nihilanth attacked with powerful energy attacks and by teleporting the doctor into different areas bristling with ferocious creatures. Gordon was not dissuaded and fought on until he was finally able to defeat the Nihilanth by firing into its brain, causing the creature to violently explode. Upon the Nihilanth's death, the dimensional rift began to collapse.
The Nihilanth is evidently an ESPer, wielding both telepathic and telekinetic power. It gains energy from Xen crystals and uses this energy to maintain the portals between Xen and Earth. When battling Gordon Freeman, it communicates via telepathy and attacks by casting blasts of mental energy. It also shields itself with this energy, conjuring golden spheres that protect it from damage. Freeman is only able to lower the Nihilanth's defenses by destroying the Xen crystals within its lair. Once the spheres protecting it are gone, the Nihilanth's head opens like a blooming flower, revealing its brain and allowing Freeman to attack it directly.