The Imprisoned One was a Rakata who lived during the Infinite Empire's dominance over the galaxy. His quest for personal power resulted in his own kind imprisoning his consciousness in a mind prison. His consciousness vessel was eventually rediscovered twice: first by Revan during the Jedi Civil War, and again several centuries later by the Czerka Corporation during the Cold War.
The mind prison in which the Imprisoned One was contained
The Imprisoned One lived sometime during the height of the Infinite Empire. Despite the numerous, galaxy-spanning accomplishments of his race, he sought to enhance his own prestige beyond that of any Rakatan. Seeking to use the Force and some kind of nanotechnology to assure his own immortality, he quickly became regarded as a criminal and a heretic amongst his own kind. They hunted him down and, by means of other technologies lost to history, extracted his consciousness from his physical body to be placed in a mechanical vessel. The vessel was placed deep in a cave, to become his eternal prison.
With the eventual fall of the Infinite Empire, he lay dormant and forgotten in his prison for many thousands of years. Eventually he was re-discovered and interacted with by Revan during his second search for the Star Forge when he opened the prison against the warnings of Lurze Kesh.
The Imprisoned One
The Imprisoned One knew how to perform a ritual that could release another from the Rakatan Prison. He made a deal with Revan, the most recent to enter the endless void. If Revan could defeat him in a game of riddles, he would be free. However, if Revan lost, the prisoner would get out and inhabit Revan's body, dooming him to an eternity in the box. After a few riddles, Revan stumped him. The prisoner stayed inside and reluctantly released Revan, helping the hero along with his quest for the Star Forge.
During the Cold War, three centuries later, the Imprisoned One was discovered again by a team of Czerka Corporation archaeologists, whom he quickly turned into unthinking drones by means of the very nanotechnology he had used to attempt his own immortality millennia earlier. Emergency protocols executed by the Czerka staff were effective in isolating the cave and preventing the spread of the Imprisoned One's infection, but the damage was done: once forgotten to the sands of time, the modern era was once again aware of his existence.
Some time later, the Imprisoned One's infection once more spread past his cave. Eventually it came to the attention of both the Imperial and Republic forces on Tatooine when a number of individuals became infected and turned on their co-workers.
The Imprisoned One first appeared in the Star Wars: Knights of the Old Republic video game. The riddle game he plays with Revan seems to be inspired by chapter five of J.R.R. Tolkien's The Hobbit: Riddles In The Dark. In this chapter, Bilbo Baggins, the titular Hobbit, engages in a Riddle-game with a being called Gollum in order to avoid being eaten. Indeed, one of the riddles used by the Imprisoned One was first used in The Hobbit by Gollum. This riddle was, "What has roots as nobody sees, is taller than trees, up, up it goes, and yet never grows?"
After appearing in Knights of the Old Republic, the Imprisoned One appeared again in BioWare's 2011 massively multiplayer online role-playing game follow-up title, Star Wars: The Old Republic. Finding and dealing with the Imprisoned One is the climax of the Tatooine story arc for both the Republic and the Empire. Republic players are given the option of destroying the mind prison to prevent the Imprisoned One from ever being freed (light side), or giving it to a rogue Czerka agent to exploit (dark side). Empire players can also destroy the mind prison (light side) or turn it over to the Imperial Reclamation Service for exploitation (dark side).