Tieran was a strong-willed male who became Autarch of Ilari in the 22nd century. Tieran, after being 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. His goal was to reclaim the ancient Ilari throne and rule the Ilari world forever. When he was beamed aboard USS Voyager, after being rescued from a disabled starship, his host body died but not before he transferred his consciousness to Kes.
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, whose own mental powers combined with Kes' mental powers, 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. Ater 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. Tieran then sent a message to 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 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. But he underestimated Voyager, which sent a landing party and attacked the palace. 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 destroyed. (VOY: "Warlord")