Tieran was a male Ilarian who became Autarch of Ilari in the late 22nd century. While initially being a uniting force for the Ilarians, Tieran soon became manipulative, power hungry, and extremely ruthless. Eventually, his terrible reign was finished when he was overthrown.
Although Tieran was killed, he had developed a way to transfer his consciousness into another living host, and had the ultimate goal of one day regaining the Ilarian throne and once again becoming ruler of Ilari. Around two hundred years later, Tieran was transported aboard the Federation starship USS Voyager after being rescued from a disabled vessel. Whilst his host body died shortly after his arrival, Tieran managed to transfer his consciousness into the Ocampan nurse, Kes.
In Kes' body, Tieran returned to Ilari and staged a coup d'état which saw him take control of the government and once again become Autarch, after killing the previous Autarch and taking one of his son's prisoner. However, Tieran found that taking over Kes had mixed blessings as his mental powers combined with hers meant that he increased perception and abilities, but it also meant that he was constantly fighting with Kes' consciousness for control of her body.
Eventually, Tieran's consciousness was removed from Kes when an away team from Voyager were able to use a synaptic stimulator. Tieran was then killed when he attempted to transfer his consciousness to one of his aides. (VOY episode: "Warlord")
Unfortunately, the aftereffects of Tieran's invasion on Kes resulted in some radical changes in her behavior following the event. While Tuvok worked with Kes to try and hone her mental abilities and drive out any afterthoughts, Kes found it difficult to meditate, and would also feel intense anger and frustration on occasion, especially with her Talaxian partner, Neelix. Fortunately, Kes managed to found comfort and clarity after working with B'Elanna Torres on constructing a windmill on the holodeck. (VOY short story: "Winds of Change")