Tieran

From Memory Alpha, the free Star Trek reference.

Tieran was a strong-willed male who became Autarch of Ilari in 2173. Tieran, after being a uniting force for his people, became manipulative, power-hungry, and ruthless. He was eventually overthrown by the previous autarch's relatives.

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. 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.

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. Voyager sent a landing party and Neelix was able to place a synaptic stimulator on Kes that drove Tieran out of her. Tieran transferred to Ameron, but Kes placed the device on him and Tieran was destroyed. (VOY: "Warlord")

Tieran was played by Leigh J. McCloskey.