The Ilari were the natives of the planet Ilari in the Delta Quadrant. They were warp-capable humanoids with a rigid system of government. Their highest office was that of Autarch, which was marked by a talisman. Other important positions included the First Castellan and regional viceroys controlling territories. The Autarch controlled a fleet of spaceships.
They were unfamiliar with transporter technology.
Sometime in the 22nd century, the Autarch was a man named Tieran. He came to power at a time of crisis and ruled well. However, as a peacetime leader, he became paranoid and dangerous. He spent most of his time, and Ilari's resources, trying to find a way around his mortality, and eventually succeeded in developing a technology that allowed him to move between host bodies.
Eventually, a rebellion occurred. Siege was laid to the Imperial Hall for over a year and the city around it was burned to the ground, but ultimately Tieran was defeated. After this, descendants of the leader of the rebellion held the position of Autarch.
In 2373, after years of planning, Tieran staged a coup attempt. USS Voyager became involved in the conflict as shortly before, Tieran had died there and took over the Ocampa Kes. Although he found her physically weak, he discovered that he was able to put her psychokinetic abilities to dangerous uses and grew fond of her body.
Tieran quickly returned to Ilari, and, despite concerns from his advisors, went ahead with the coup. He quickly infiltrated the main chamber of the Imperial Hall and killed the ruling Autarch. He then persuaded the dead leader's younger son, Ameron, to join him. After taking power, Tieran raged that corruption had become too commonplace on the planet, contrasting it to his own "moral" leadership. He also ordered the construction of a library and issued an edict that every citizen must have a garden.
As a civil war broke out on the planet, Tieran worked to forge alliances and raised a fleet to counter one assembled by forces loyal to the former Archon. The crew of Voyager rescued Kes but failed to prevent Tieran from taking Ameron as a new host. Using a piece of technology designed by The Doctor, they removed Tieran's influence from Ameron and left the planet, leaving the old Autarch's elder son, Demmas, in power. (VOY: "Warlord")