One of the more mysterious species the Federation has encountered, the Talosian culture dates back hundreds of thousands of years. In the distant past they waged a terrible war which rendered the surface of their planet uninhabitable, forcing the few Talosian survivors to retreat underground. They found this life highly restrictive, and so devoted themselves to developing a tremendous level of telepathic ability. They are able to project utterly convincing images into the brain of another species across distances of at least several light years, apparently at speeds greatly in excess of light. However, this ability proved to be the downfall of their race. The Talosians enjoyed sharing the experiences of those aliens they had collected prior to the war, but this became a dangerous narcotic for them. They came to rely on their fantasy life more and more, eventually losing almost all touch with reality. Meanwhile, their technology crumbled and decayed around them to leave them all but helpless.
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The Talosian response was to bring aliens to their world, hoping to select appropriate specimens and use them to breed an entire race of slaves to maintain their society whilst they continued to enjoy their telepathic fantasy lives. The first encounter between Talosians and Humanity came when the SS Columbia crashed on Talos IV in 2236. All the crew were killed except for one woman, Vina, who was badly injured. The Talosians repaired her injuries, but although they found Humans to be an ideal physical choice for their slaves they were not able to proceed with their plans for a further 18 years due to the lack of any male candidate.
In 2254 the USS Enterprise, under the command of Captain Pike, encountered a radio distress call sent by the Columbia and proceeded to Talos. The Talosians kidnapped several crewmembers, including the Captain, in order to start their slave race. Pike made every effort to escape, but the Talosians mental abilities made it all but impossible - so complete were their abilities that they could easily destroy the starship just by tricking the crew into pressing the wrong buttons.
Ultimately, Pikes first officer threatened to kill all of the captives rather than submit to slavery. Realizing that the Humans would never accept captivity, the Talosians allowed them to leave. 
Starfleet quarantined the planet, forbidding any Starfleet ship to approach under penalty of death for its Captain. Yet in 2267 the USS Enterprise was hijacked by its first officer, Spock, and taken to Talos. Captain Pike had recently suffered devastating injuries from exposure to delta rays, leaving him permanently crippled. Spock was convinced that Pike could live out the rest of his days more comfortably on Talos, where his mind could be convinced that his body was once again healthy. Starfleet ultimately allowed the mission due to the special circumstances. 
Since this time there has been no further contact with the Talosians. Presumably their society is still decaying, although the presence of a Human male and female may mean that a small group of Humans was produced after all.