Tieran was a strong-willed male who became Autarch of the Ilari in the 22nd century. According to Tieran himself, he was a sickly child who doctors believed would not live to adulthood. Tieran was an able military leader who after shepherding his people through difficult times, serving as a uniting force for his people, became manipulative, power-hungry, paranoid and ruthless. He was eventually overthrown; the new Autarch's descendants still ruled as of 2373.
He was able to continue to exist for over two hundred years by transferring his mind to host bodies via a sophisticated neural implant. His goal was to reclaim the ancient Ilari title and rule the Ilari homeworld forever. When his current host was beamed aboard the USS Voyager, after being rescued from a disabled starship, his host body died but not before he transferred his consciousness to Kes who was tending to the host body's injuries in sickbay.
In Kes' body, Tieran returned to the planet and staged a coup that toppled the government. Tieran killed the then-current Autarch and abducted one of his sons, Ameron. Tieran worked to build alliances with various people by exchanging favors and was not above using sex to get what he wanted. In Kes' body, he attempted to seduce Tuvok and resume his marriage to his wife Nori.
With his mental strength combined with Kes' mental powers, Tieran became extremely powerful, to the extent that he could inflict pain on others through mental telepathy, as he did to Resh, his general, who doubted him due to the apparent weakness of his current host before he demonstrated Kes's powers.
These enhanced powers had side effects. Kes continued to fight Tieran for control of her body, resulting in a series of severe headaches and bouts of split personality as Kes tried to gain control. After Tuvok was captured in a failed rescue attempt, Tieran interrogated Tuvok trying to use Kes' mental powers to get the information he desired. This failed when Tuvok was able to initiate a mind meld and speak directly to Kes, who told Tuvok she was fighting Tieran for control. Tuvok expressed his confidence in Kes and assured her that he was fighting alongside her until Tieran broke the link.
Tieran then sent a message to the Voyager informing them to stay out of his affairs, owing them for saving his life. Afterward, Tieran hosted a celebration for his marriage to Ameron (which he considered a political alliance for his benefit). But when the Voyager and a number of loyalists advanced on the capital, Tieran refused to hide in a bunker and ordered everyone to celebrate rather than fight. However, he underestimated the Voyager crew, who sent a landing party and attacked the Imperial Hall. With Chakotay providing cover fire, Neelix was able to place a synaptic stimulator on Kes, but not before Tieran had transferred to Ameron. Kes placed the device on Ameron and Tieran was killed. (VOY: "Warlord")