The Door Into Fire(1979)

Diane Duane

Herewiss, Prince of the Brightwood, is the only man in centuries to possess the Power of the Flame, but he cannot use or control it, not even to help his dearest friend, Freelorn, the exiled Prince of Arlen. But Herewiss does have a talent for sorcery, and, aided by the enigmatic creature Sunspark, he is able to rout the armies besieging Freelorn.

Now Herewiss faces a devastating choice. With or without the Power, he knows his time is running out. Shall he join Freelorn in his fight to regain his kingdom? Or shall he seek out the ancient castle where doors lead to other worlds, worlds Sunspark has hinted at, perhaps even the door that would teach him to control the Power...