Xen Controller

Controllers are semi-sentient alien creatures originating from the borderworld of Xen in Half-Life. They appear vaguely humanoid, though they have a third arm extending from the chest and extremely large heads. They closely resemble the Nihilanth and may be the same race.

Overview
The Controllers share many characteristics with the Nihilanth, suggesting that they may be closely related. Like other sentient creatures on Xen, the Controllers have a third vestigial arm growing from the centre of their chest. In addition to this, the Controllers have disproportionately large heads, which can peel open to reveal a spike or cone, resembling the interior of the Nihilanth's skull. Unlike the Nihilanth, the Controllers have longer legs and are able to levitate without the assistance of a mechanical seat. It is unknown whether this ability stems from their strange psychokinetic powers, or from their seemingly mechanical parts. The Controllers may have simply died out after Nihilanth's death.

Behaviour and skills
Despite their oversized head, and spectacularly-grown brain, Controllers' intelligence is very limited in several aspects. They can not communicate with each other, or any different kinds of specimen on Xen. Unlike their master, they can't send telepathic messages to Gordon Freeman. Their language is limited to babbling, and various low- and high-pitched screeches and whines when noticing a threat.

They attack by firing multiple spheres of electrical energy (resembling ball lightning) from their hands, in the same way as the Nihilanth. They can also fire an energy ball from their head that is larger than, and more powerful than the spheres from their hands, and that has a slight homing ability.

Controllers are usually encountered near places where Vortigaunts are made to work. They seem to have had an unspoken influence over the Vortigaunts, making them fight or toil at a whim. This might indicate that the Controllers share a limited version of Nihilanth's psychic enslaving power or act as an intermediary conduit, although their exact role in the hierarchy is unknown.