Shadowsong by S. Jae-Jones

Six months after the end of Wintersong, Liesl is working toward furthering both her brother’s and her own musical careers. Although she is determined to look forward and not behind, life in the world above is not as easy as Liesl had hoped. Her younger brother Josef is cold, distant, and withdrawn, while Liesl can’t forget the austere young man she left beneath the earth, and the music he inspired in her.

When troubling signs arise that the barrier between worlds is crumbling, Liesl must return to the Underground to unravel the mystery of life, death, and the Goblin King—who he was, who he is, and who he will be. What will it take to break the old laws once and for all? What is the true meaning of sacrifice when the fate of the world—or the ones Liesl loves—is in her hands?