Soul of Glass and Fire (Soul Erosion #2) by Arianne Nicks:
 
*This book has a content warning for readers please check our details and the author's resources for details.*
 
The second book in the Soul Erosion series brings answers to the most ardent questions from its prequel, Soul of Light and Thunder. But new mysteries arise.

In the chaos of the tournament’s last battle, Sophie loses her half of the mirror when her worst enemy, Raiden, relocates them both to an unknown place.
Sophie discovers a reality-altering letter that only Raiden can explain. And next to his unconscious body is the other half of the mirror.

To find out the truth, Sophie needs to heal her rival. But what terrifies her more than bringing to life the man she fears most, is the possibility of his letter being genuine.

Sophie’s world is wavering. She has either loved and hated the wrong people, or she will be dead as soon as Raiden wakes up.