Zar-Tan was an artificially-created being—technically a cyborg—that began attacking the Terralyn colony on Terrellian II on just before Stardate 2314.6.
He piloted an omni-orbital class C star cruiser an wore armor that resisted sensor scans. Zar-Tan stated the planet belonged to him, and was compeled to destroy those he considered "trespassers." Spock, with the help of the projection called "The Ghost," restored Zar-Tan's damaged memories of his true mission. He had been created by the Thelemites as a storage vessel for their genetic patterns. With the truth revealed, Zar-Tan was deactivated, and the organic patterns he contained were used to restore the Thelemite people. (TOS comic: "The Empire Man")