The Maelstrom's Heart by Adam K. Watts

Separated from her companions and enslaved, Mira must come to grips with her new reality.

Reeling from the loss of those closest to her and blocked from using magic, she must fight in the ring every day just to survive, magically healed from her wounds only to be forced to fight soon again.

Meanwhile, the time spell is spinning out of control, threatening to destroy all of Daoine. Some believe Mira is the only one who can do anything about it. But has she become too broken to care?