Nihilanth

The Nihilanth is the final boss character and the primary antagonist in the first edition of Half-Life.

The Nihilanth is the controlling intelligence behind the invasion of Earth. Hovering silently in an enclosed cavern dozens of stories tall, it has been commanding the forces of Xen and holding open a dimensional rift connecting Xen to Earth, enabling its creatures to attack.

Physically, the Nihilanth resembles a gigantic, oddly-proportioned fetus with its head alone outsizing the rest of the body, and its brain cavity forming most of the head's bulk. Its legs, minuscule in relation to its body, appear either to be vestigial, or are the remains of amputation. The creature sits on a large mechanical levitating contraption consisting of a large base with spikes underneath. Like the rest of the intelligent Xen races, the Nihilanth has a third appendage protruding from the middle of its pectoral chest region. Its unusually long arms end in disproportionately huge hands and fingers, and it wears metallic wristbands, which look similar to bands worn by the Vortigaunts; contributing to the evidence that the Nihilanth was once a slave to an obscure higher power (presumably the Combine, as the Nihilanth moved its army to Xen to escape them) just as it is a master to the Xen races.

The Nihilanth attacks by launching energy balls, which come in two types: a swarm of blue-colored electrical orbs that causes immense damage, and a slow-moving, green-colored orb (similar in appearance to the teleportation portals constructed in the Black Mesa Research Facility) which homes in on and teleports its target to other sections of its lair populated by Xen aliens. It also uses these green orbs to teleport Vortigaunts and other Xen creatures into its main chamber to help defend it against intruders. This may not be intended as the creatures appear only when the green orb flies into a wall or rock or is destroyed by the player and triggers the teleportation. A series of gold-colored "energy spheres" orbit around the Nihilanth's head, rendering it impervious to damage. These are consumed as the Nihilanth attacks or summons assistance, but specialized crystals within the upper portion of its chamber replenish the spheres as they vanish. If some determined invader destroys these crystals and does sufficient damage to its body, the Nihilanth's head opens up like a flower, revealing a shining, vulnerable brain. Sufficiently damaging its brain destroys the Nihilanth. Additionally, once the Nihilanth uses up all it's golden energy orbs, and cannot replenish them, it can only attack with one electrical orb, instead of a swarm of ten, or a teleportation orb.

Gordon Freeman was tasked with killing the Nihilanth because it was the only force holding the dimensional rift open after the satellite delivery rocket was used by the Black Mesa Lambda Labs to reverse the resonance cascade. It is believed that the Nihilanth's death eventually triggered the portal storms.