On this Broken Earth (The Grail Cycle) by Kate Schumacher

For the serpent to become the dragon, it must consume itself and be reborn. The search for the Treasures of the Gods leads Arthur, Ordes, and Jenyfer to Malist and the seat of the One God. There, with the help of a mysterious Red Sister, they learn more about the hands that turn the wheel of Merlin’s prophecy. In Kernou, Lamorna has to choose between what she believes and what she knows to be true. With Mordred by her side she begins to find her voice and an agency she never dreamed existed. Finally, power is hers to wield. In their pursuit of the Treasures, the trio must choose to face their deepest fears. But when betrayal forces Arthur’s hand, the stage is set for a confrontation that will shape the world – while the weight of prophecy threatens to sink them all. On this Broken Earth is the third book in The Grail Cycle, an epic, romantic fantasy reimagining of Arthurian legend.