Alien Controller

An Alien Controller (also known as a Xen Master) is an enemy which appears in the computer game Half-Life. Physically, the Controllers are small, levitating creatures which closely resemble the Nihilanth. In Half-Life, they do not appear in the Black Mesa Research Facility until Gordon Freeman is about to teleport into Xen, and they are more numerous in the alien world itself. They appear much more frequently within the complex in Half-Life: Decay during moments of increased displacement activity.

Description
The Controllers share many characteristics with the Vortigaunts, Gargantuas, Alien Grunts, and especially the Nihilanth, suggesting that they are all closely related. Like these other sapient species on Xen, the Controllers have a third vestigial arm growing from the center of the thorax. In addition to this, the Controllers have disproportionately large heads, which can peel open to reveal a (apparently mechanical) spike or cone. They attack by firing fiery spheres from their hands, in the same way as the Nihilanth. They can also fire an energy ball from their head that is larger, more powerful and has a slight homing ability. Although these attacks resemble flame, they are actually spheres of electrical energy, like ball lightning. Unlike the Nihilanth, the Controllers have long 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.

During the Black Mesa incident on Earth, Controllers only appeared very rarely. As such, Controllers are found almost exclusively on Xen, usually encountered near places where Vortigaunts are made to work. Controllers seem to have 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.