A Wizard in War — Christopher Stasheff

Magnus D'Armand, renegade son of Rod Gallowglass, The Warlock in Spite of Himself, has inherited his father's overdeveloped sense of justice, along with his awesome psychic gifts. But unlike his father, Magnus is unwilling to play by the rules, choosing instead to single-handedly seek out oppressed planets to liberate, and sow the seeds of revolution throughout the galaxy.

The Third Chronicle of Magnus D'Armand

The medieval planet of Maltroit seems to be repeating Earth's bloody history in the worst possible manner. It's up to the Rogue Wizard to give the downtrodden a crash course in democracy, so before you can say "Magna Carta," he's bent the course of history by teaching the nobles the rudiments of democracy, and schooling the peasants in the basics of radical politics.