Loremaster: Dragon’s Key (Ascension of a Street Rat, Book 3) - M. E. Robinson

Battered and broken, Rowan once again refuses to die. Deep within the bowels of the earth, he wakes to find himself cut off from the world, his mana paths severed, and trapped within a strange dungeon: the Dracoludus. Guarded by an ancient dragon who holds the only key to his escape, Rowan must grow stronger, and fast. For even if his friends survived the events of Miriarda Forest, then they are still in mortal danger, stalked by a genocidal cult who will stop at nothing to see their god returne