Heart of the Sun Warrior by Sue Lynn Tan , Kuri Huang

After her perilous quest to free her mother, Xingyin thrives once more in the tranquility of her home. But her fragile peace is threatened by the discovery of a strange magic on the moon and the unsettling changes in the Celestial Kingdom as the emperor tightens his grip on power. While Xingyin is determined to keep clear of the rising danger, the discovery of a shocking truth spurs her into a treacherous confrontation.

Forced to flee her home once more, Xingyin and her companions venture to unexplored lands of the Immortal Realm, encountering legendary creatures and shrewd monarchs, beloved friends and bitter adversaries. With alliances shifting quicker than the tides, Xingyin has to overcome past grudges and enmities to forge a new path forward, seeking aid where she never imagined she would. As an unspeakable terror sweeps across the realm, Xingyin must uncover the truth of her heart and claw her way through devastation--to rise against this evil before it destroys everything she holds dear, and the worlds she has grown to love... even if doing so demands the greatest price of all.