This article is about the creature. For the Half-Life chapter, see Nihilanth (chapter).
During the Black Mesa Incident
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The Nihilanth is a massive, floating being that resembles an abnormally proportioned humanoid fetus, with a large head atop a smaller body. It shares many physical traits with other intelligent Xen life forms, especially with the Alien Controllers, and is related to the Vortigaunts, as indicated by their vestigial chest-arms, which is why it had a particular ability to control them. It also wears shackles similar to those worn by enslaved Vortigaunts. The Nihilanth's body features what appears to be the remains of some sort of surgical procedure, and its minuscule legs appear to be either vestigial or the remains of amputation.
Some of its backstory was revealed in a series of conversations with the developers. According to Doug Lombardi (restating something that was told him by Marc Laidlaw), the Nihilanth's homeworld was invaded by the Combine long before the events of Half-Life, prompting the migration and takeover of Xen by the Nihilanth. The Nihilanth, the Vortigaunts, and the Alien Grunts were all from the same world. It was also said the Nihilanth was last of its kind, and some of its predecessors might have been captured by the Combine.
 Behavior and skills
The Nihilanth appears to be floating in a meditation pose. It has two attack moves. The first attack is a stream of purple-colored energy projectiles, similar to the Alien Controller's fireballs, but stronger. The second attack is a large green sphere that teleports the player to other areas populated with aliens. These spheres can be destroyed by the player. The Nihilanth can also summon Vortigaunts and Alien Controllers for help in the battle.
A series of orange-colored "energy spheres" orbit around the Nihilanth's head, and provide it with energy. It uses this energy to protect itself from harm and to attack the player. When this energy runs low (after being attacked by the player), the Nihilanth replenishes the spheres by absorbing energy from one of the crystals in the chamber.
When the crystals are destroyed, the Nihilanth becomes vulnerable, and its head opens up like a flower after some time, revealing a large energy sphere atop a fleshy prong inside his head, which is its only weak spot. Eventually, the Nihilanth floats up with beams and green spheres coming out of its body and dies in a massive green explosion.
The scientists suspect there is an immense portal at the borderworld, created by the intense concentration of a single powerful being. Gordon Freeman is sent to kill the Nihilanth to stop the invasion, and close the dimensional rift between the worlds. The Nihilanth regularly contacts Gordon telepathically and speaks to him in English in Xen.
Dr. Richard Keller thinks that something is interfering from the other side of the rift, when Dr. Gina Cross and Dr. Colette Green attempt to reset the dampening fields to prevent another cascade. Keller says whatever held the rift open did so with intent, and aliens may not all be here by accident. Furthermore, in the bonus chapter Xen Attacks, the Nihilanth sends two Vortigaunts, drone subjects X-8973 and R-4913, to retrieve several stolen Xen crystals.
The Nihilanth is mentioned by Vortigaunts throughout the game in random conversations. It is revealed through these messages that the Vortigaunts see Gordon as a messianic figure after he liberates them by killing the Nihilanth. This is also the only in-game instance in the series where the Nihilanth's name is mentioned by the characters.
 Behind the scenes
Half-Life 2: Raising the Bar states that the first part of the Nihilanth's name is reference to nihilism (from the Latin nihil, meaning nothing), while -anth is a common suffix that is related to flowers and is a reference to how his scalp peels open like a blossom. Michael Shapiro is uncredited to the Nihilanth's voice. However, it can be verified by listening to the Nihilanth's speech sounds and by changing the speed of its grunting sounds.
 List of appearances
 Main games
- Half-Life (First appearance)
- Half-Life: Decay (Mentioned only) (Heard only)
- Half-Life 2 (Mentioned only)