The Tale of the Five Series by Diane Duane

Herewiss is the only man in centuries to possess the Power of the blue Flame, but he can't use or control it -- not even to help his dearest friend, Freelorn, exiled prince of Arlen. Herewiss does have a talent for more mundane sorcery, and (aided by the unearthly creature Sunspark) he uses it to rout the armies besieging Freelorn. But now Herewiss faces a devastating choice.His time to master the blue Fire is running out... moreHerewiss is the only man in centuries to possess the Power of the blue Flame, but he can't use or control it -- not even to help his dearest friend, Freelorn, exiled prince of Arlen. Herewiss does have a talent for more mundane sorcery, and (aided by the unearthly creature Sunspark) he uses it to rout the armies besieging Freelorn. But now Herewiss faces a devastating choice.His time to master the blue Fire is running out. Should he join Freelorn in his fight to regain his kingdom? Or should he seek out the ancient keep in the Waste where doors lead into other worlds -- perhaps even the door whose use will teach him to control the Power that he must master or die?... less

As the eternal malice of the Shadow rises once more to threaten Creation with destructions, Freelorn the exiled prince of Arlen stands with four others to confront this looming apocalypse. Reissue.

Only Freelorn, the banished Prince of Arlen, can rescue the dying land of Arlen from the usurper king and the evil that threatens it, by rediscovering a missing ritual, lost at the time of Freelorn's father's murder. Reprint.