Shattered Lives. The Salvation of Tempestria, #6

As the Guardians' quest to find the dangerous, unstable fragments comes to a head, they have learnt that their greatest strength is not their magic but their unbreakable bonds of friendship. Their enemy knows it, too, and tries to divide and conquer. The Guardians have a ridiculous radical plan to identify their mysterious enemy, but it hinges on a girl who has sold her soul and believes the White Guardian murdered her father.
To save the world, they must first save her soul. But their greatest threat comes not from without but from within, and it is not the magic of enemies but the secrets of friends that have the power to shatter their lives.