While it is a race, the Nihilanth that appears within Half-Life is the final member of it's race, who enslaved Vortigaunts who escaped to Xen after it itself had escaped there. It is the leader of the Xen army that attempts to take over the planet Earth. It is a monster with a grotesquely large head, who is missing half of it's torso, along with three fingers.
The Nihilanth is the final boss of Half-Life, in which Gordon Freeman must kill it.