Secrets of the Undercity — E.A. Rappaport

Abandoned and betrayed as a child, Halia seeks revenge. Frustrated by his limited abilities, Oswynn seeks powerful magic. The paths of the thief and apprentice cross when battling an eruption of dangerous half-breed creatures that might destroy an entire kingdom. Halia seeks revenge against the Terun thief who taught her to steal when she was a child and then framed her for his crimes. But her personal vendetta is sidetracked when her friend, the hapless apprentice Oswynn, accidentally releases thousands of dangerous creatures from a mystical prison. One of them, a vicious half-dragon, conspires with Halia's archenemy to create the ultimate army of bizarre crossbreeds, including flying sabertooths and stone-skinned wolves. Despite lies and betrayals, Halia must find allies on a cursed island to help her defeat the legions of unearthly beasts before they destroy civilization.