Astrid Llewelyn has always scoffed at her eccentric mother's stories about killer unicorns. But when one attacks her boyfriend—ruining any chance of him taking her to prom—Astrid finds herself headed to Rome to train as a unicorn hunter at the ancient Cloisters the hunters have used for centuries. However, all is not what it seems at the Cloisters. Outside, unicorns wait to attack. And within, Astrid faces other, unexpected threats: from bone-covered walls that vibrate with terrible power to the hidden agendas of her fellow hunters to her growing attraction to a handsome art student . . . an attraction that could jeopardize everything.