The Zetars, a.k.a. Zetarians, are an intelligent race which used to inhabit the planet Zetar, until a series of natural cataclysms destroyed their corporeal existence. After that, the consciences of the last hundred members of the species managed to remain alive non-corporeally and roamed through the cosmos for a millennium, searching for a compatible physical body that they could inhabit.
The non-corporeal Zetarians are said to embody the desires, the hopes, the mind and the will of those who refused to accept their deaths during the catastrophe. Their one goal is to find a way to continue their existence, even at the cost of others.
Being ethereal entities, the Zetarians are capable of incredible feats such as traveling through space at warp speed and interfering with the working system of computers. While they can possess humanoid bodies, most aren't fully compatible with them and they're thus rendered incapable of seeing, speaking, hearing or moving. Attempts to resist the Zetarians' mental presence for too long usually prove fatal for the host.
While inhabiting a humanoid body, the Zetarians can be killed by a sufficient increase in atmospheric pressure. In 2269, Captain Kirk and his officers successfully employed this method to save the life of Mira Romaine, who had been possessed by the Zetarians. It's likely that the species has been rendered extinct afterwards.
- Star Trek, s03e18, "The Lights of Zetar" (1969)