Lunar is missing, she left a note to tell everyone not to look for her, she had no choice. Leaving Elias sleeping in her bed, she fled the house and her life to work for Charon in the Underworld. He threatened to kill everyone she loved, so she had do as he asked.
Charon wants her to collect souls for him, he has used his manipulative powers to influence her decision and it fast becomes something she can't bear to live with.
The realms are in chaos, with the most powerful collector missing, the balance is out of whack and all of the realms must work together to bring her home safely.
Her prophesy is waiting, Sheol is waiting, Elias is waiting too, time is running out, losing her to the Underworld forever, is not an option.