Marion Zimmer Bradley's
Darkover

Darkover, the planet of the bloody sun, was once, it is thought, a Terran colony, founded by one of the lost generation starships from the earliest days of interglactic colonization. That ship crashed, its computers and navigational systems completely destroyed. Now, 2000 years later, the Darkovans have created a complex, basically feudal culture on their harsh, cold planet. The ruling caste has telepathy--an ability created by interbreeding with a dying alien culture and intensified by breathing the mind-altering pollen of a flower called Kireseth. Having developed their abilities through generations of selective breeding, they have even made a science of their magic, discovering the uses of blue matrix crystals in focusing and amplifying their mental strengths. Theirs is an insular, conservative culture of two thousand years. And then, finally, the Terran Empire catches up with their lost colony, and wants to introduce the inhabitants of Cottman IV (which the natives know only as Darkover) to intergalactic society...

The Darkover Series