The Ilari are the natives of the planet Ilari in the Delta Quadrant. They are warp-capable humanoids with a rigid system of government. Their highest office is that of Autarch, who is marked by a silver Collar of Office.
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. Eventually he was ousted from power but not before he had developed a technology that allowed him to move between host bodies, making him theoretically immortal.
In 2373, his host body died aboard the USS Voyager and he 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. Returning to Ilari, he staged a coup and killed the ruling Autarch. He then persuaded the dead leader's younger son, Ameron to join him.
As a civil war broke out on the planet, the crew of the 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")