Academy of Thaumaturgy: Book Four

Ari Li was never destined for magic. But destiny had other plans. As a Mundane in a world brimming with Thaumaturgy, Ari's path to becoming a Sentinel-a guardian sworn to protect the Light from the forces of Devilry-was all she ever wanted. Years of grueling combat training had prepared her for the battles ahead, but nothing could have prepared her for the devastating loss of her closest friend, her sister in all but blood.
Driven by vengeance, Ari vows to hunt down the Master responsible, even if it means turning against the Light she once swore to protect. When a sinister artifact is unleashed, trapping her in a twisted prison, Ari is forced to strike a bargain with an unexpected ally: a Shadow Fae whose formidable power has been sealed away for centuries. But bargains with shadows always come with a price. As Ari wrestles with the Shadow Qi threatening to consume her, she realizes the darkness isn't just inside her-it's targeting those she holds dear, including Viggo, the Necromancer who might be the only one capable of saving her.
With the fate of her soul hanging in the balance, Ari must decide: will she embrace the light and its fragile hope, or give in to the shadow for the power to destroy her enemies?