The Kohl were a race from the Delta Quadrant. In 2353 their world, which was a major trading post, was almost destroyed by a solar flare. Five of the Kohl entered hibernation using an interactive simulation to keep their minds active.
Unfortunately the simulation tapped into their subconscious minds and created an entity from their fear, known as "the Clown." The Clown tortured them for 19 years before being discovered and destroyed by the crew of the USS Voyager. Sadly only two of the Kohl had survived. (VOY: "The Thaw")