The Vians were about the same size as Humans, their hairless heads were somewhat triangular and feature several prominent ridges. They had grayish skin, and were known to wear long robes made of a metallic fabric. (TOS: "The Empath")
Technologically, the Vians were extremely advanced by 23rd century Federation standards. They possessed the capability of transporting an entire population of one planet to a safe location. They also possessed the ability to create detailed mirages of people they had never met and appeared to have the ability to read the minds of humanoids. They also were somewhat resistant to the Vulcan nerve pinch. When employed against them, they were alert again within seconds.
Each Vian's energy transfer device responded to its owner's thoughts, and no others. The devices could be altered to another thought pattern only with great difficulty. Doing so required a working knowledge of the desired thought pattern and a good deal of time. Once attuned, it could alter the environment around the user, transfer the user or others from point to point in the manner of a transporter, inflict or heal injuries, repel individuals, alter the rate at which time progresses, and generate a type of force field that fed off the energy and emotions exerted by those contained within. Spock was eventually able to suppress his emotions and free himself, but Kirk could not achieve the necessary detachment. Many of the effects were preceded by a flash of light and/or accompanied by a sound. Spock theorized that the flash of light might be the device establishing a link to a remote energy source. (TOS: "The Empath")
In 2268 the star Minara was about to become a nova. The USS Enterprise visited a scientific outpost studying the star to retrieve the scientists and their data. They discovered the outpost abandoned. Before they could investigate further, Vians captured Kirk, Mr. Spock, and McCoy and subjected them to various torments as part of an arcane plan to awaken the instinct for self-sacrifice in a member of another species. They demonstrated a willingness to sacrifice any number of people in pursuit of this goal. Kirk eventually won freedom for himself and his crew by showing the Vians that they had lost the ability to feel the same benevolent emotions they wished to awaken in their experimental subject. (TOS: "The Empath")
In 2269, Kirk, whose mind was possessed in the body of Janice Lester, attempted to prove to Spock his true identity, explaining how, during their encounter with "the Vians of Minara demanded that we let Bones die, [but] we didn't permit it." Spock initially explained that those events were recorded, and that "They could have become known to you." (TOS: "Turnabout Intruder")