While physically weak, the Talosians had superior telepathic abilities that allowed them to read minds and create illusions that felt real.
They encountered Humans for the first time in the early 23rd century when an Earth starship crashed on the surface of their planet. Among the wreakage was one survivor, a Terran woman named Vina. The Talosians eventually lured the ISS Enterprise with the intention of having Captain Christopher Pike mate with Vina in order to create a race of Terran slaves who could then reclaim the devastated surface of the planet.
However, upon learning of the Terran Empire's fear for telepaths, the Talosians decided to release Captain Pike who would then act as their eyes and ears within the empire. The Captain would warn them should the empire ever chose to attack their homeworld. Not only did the Talosians maintain a telepathic link into Pike's mind once he left the planet, they also gave him the ability to create illusions, specifically the ability to maintain his youthful appearance in spite of advancing age and debilitation.
Upon returning to Spacedock, Pike recommended to Starfleet Command that Talos IV be quarantined; nevertheless, the Talosians did not trust him and considered the Captain to be weak and unsuitable for their purposes. When Lieutenant Commander James T. Kirk assassinated Pike, the Talosians attempted to link with his mind and use him as their new spy within the empire. In his first action as the new captain of the Enterprise, Kirk ordered the ship to Talos IV while he kept his mind full of savage rage so as to prevent its inhabitants from reading his thoughts.