Child of the Sword, epic fantasy by J. L. Doty

Rat is no ordinary thief. A feral, filthy and malnourished child; he haunts the streets of the medieval city Anistigh and survives on what he can steal. When he tries to steal a purse and bungles it, a mob wants to cut off his hand as punishment, so to save himself he slips into a convenient shadow to hide. Rat thinks it's just another warm and comfortable shadow cast by the sun, but a clan wizard sees him create the shadow with his magic, an instinctive act of which Rat is unaware. Rat's magic is subtle, but potentially quite powerful, so he is adopted into the greatest of the Lesser Clans, adopted into a family, and given the name Morgin.

Morgin quickly grows into manhood and the clan teaches him wizardry and sorcery and swordsmanship. Having survived the streets of Anistigh, Morgin is inclined to avoid conflict, would be content to remain on the sidelines in the ever present clan rivalries. But as a clansman he inherits the enemies of the clan, and his shadowmagic proves to be a powerful weapon. As the ancestral conflict between the Greater and Lesser Clans once again leads to war, not even the clans realize that their petty little war is spawned by the primeval battle between the righteous gods of the Celestial Plane and the fallen gods of the Nether Plane. For it is upon the Mortal Plane, with mortal lives, and mortal pain and suffering and death; it is upon the Mortal Plane that the gods meet and fight their wars.