Beyond the Shadowed Earth by Joanna Ruth Meyer

It has always been Eda's dream to become empress, no matter the cost. Haunted by her ambition and selfishness, she's convinced that the only way to achieve her goal is to barter with the gods. But all requests come with a price and Eda bargains away the soul of her best friend in exchange for the crown. Years later, her hold on the empire begins to crumble and her best friend unexpectedly grows sick and dies. Gnawed by guilt and betrayal, Eda embarks on a harrowing journey to confront the very god who gave her the kingdom in the first place. However, she soon discovers that he's trapped at the center of an otherworldly labyrinth and that her bargain with him is more complex than she ever could have imagined - he always intended for her to free him... or take his place.