Rhada #1: The Warlock of Rhada by Robert Cham Gilman, Alfred Coppel

It is the Dark Age after the fall of the Stellar Empire. The planets of the galaxy are ruled by brutal and petty warlords and Galactic culture is barely held together by the Order of Navigators, who possess the only functioning spacecraft.
On the planet Vyka, the warleader Glamiss is sent to the Trama valley to punish villagers for neglecting their yearly tribute. Little did he know that this valley and its senile Warlock would change the direction of galactic history.