Sullivan Donald H Sullivan 
Wild Magic 

Two stories: Wild Magic and The Black Star Wild Magic: At age fourteen, Carma discovers that she has magic powers. She applies for mage training and is accepted, but is soon rejected and sent home because her magic is wild and uncontrollable. Three years later, the Most High Grand Wizard sends for her and requests that she accept a highly dangerous mission. He explains that no other wizard in the kingdom can succeed in this mission, but she has a chance to succeed oddly because of her wild magic. The Grand Wizard promises that she will be accepted and remain in the mage training program until her magic is controllable. She accepts. .The Black Star: Hiram is tricked into accepting a ring with a death curse. Whoever is in posession of the ring during a full moon dies. There is only one way to get rid of the curse: someone must willingly accept the ring. To throw it away means death. Hiram has but days to get rid of the curse.