"The Outstretched Shadow," the first book in The Obsidian Trilogy from Mercedes Lackey and James Mallory
Kellen Tavadon, son of the Arch-Mage Lycaelon, thought he knew the way the world worked. His father, leading the wise and benevolent Council of Mages, protected and guided the citizens of the Golden City of the Bells. Young Mages in training--all men, for women were unfit to practice magic--memorized the intricate details of High Magic and aspired to seats on the council.
Then he found the forbidden Books of Wild Magic--or did they find him? Their Magic felt like a living thing, guided by the hearts and minds of those who practiced it and benefited from it.
Questioning everything he has known, Kellen discovers too many of the City's dark secrets. Banished, with the Outlaw Hunt on his heels, Kellen invokes Wild Magic--and finds himself running for his life with a unicorn at his side.
Rescued by a unicorn, healed by a female Wild Mage who knows more about Kellen than anyone outside the City should, meeting Elven royalty and Elven warriors, and plunged into a world full of magical beings--Kellen both revels in and fears his new freedom.
The one thing all the Mages of the City agreed on was that practicing Wild Magic corrupted a Mage. Turned him into a Demon. Would that be Kellen's fate?
Deep in Obsidian Mountain, the Demons are waiting. Since their defeat in the last great War, they've been biding their time, sowing the seeds of distrust and discontent between their human and Elven enemies. Very soon now, when the Demons rise to make war, there will be no alliance between High and Wild Magic to stand against them. And then all the world will belong to the Endarkened.