Mage's Rebellion (The Imperial Series) - Imperial, #4

Princess Calinda has always been a talented mage. As a child she astounded all her tutors. Even her formidable father, Emperor Damuk, acknowledged her talent. So much so that he trained her himself. Now Calinda is eighteen years old and a woman in her own right. All she wants is to serve the Empire like her father and brother. Unfortunately for her, there's one problem. She's a woman. Despite all her pleas, her parents will not allow her to serve in the Royal Guard.
They expect her to marry and bear children, but young Callie has no intention of doing so. Desperate to avoid their plans for her, she sneaks out in the dark of the night, never expecting the great darkness she will soon be forced to confront. As the gathering Dark threatens to consume her, she must use all her skills to avoid disaster.