Child of the Sword, Book 1 of The Gods Within

Rat is no ordinary thief. A feral, filthy and malnourished child; he survives on what he can steal. But he creates his own shadows and hides within them, though he's completely unaware of his use of magic. When a clan of powerful wizards see his shadowmagic they adopt him, because they want such magic in the clan. Perhaps that's a good thing for Rat, as long as they don't kill him in the process.