"The Ghost" was actually a thought projection from the collective effort of the Thelemites on Terrellian II on stardate 2314.6. To avoid a radioactive cloud that would destroy their civilization, the Thelemites embedded their minds into the local plant life. They planned to re-merge with their bodies—the patterns held within Zar-Tan—once the cloud had passed.
However, the mute Ghost was the only way they could communicate with Zar-Tan and the planets new occupants, the Terralyns. When USS Enterprise forced Zar-Tan's ship to crash, the Ghost became enraged and unleashed a seismic attack on the planet. Realizing her actions had injured the Terralyn chancellor, she used her powers to heal him. After merging with Spock's mind, she was able to explain the situation, leading to the restoration of the Thelemites. (TOS comic: "The Empire Man")