The Stone Sky

Essun races against time to return the Moon to a stable orbit before her daughter Nassun can use the Obelisk Gate to destroy the Earth instead. Nassun, traumatized after killing her own father and convinced the world is too corrupt to save, is guided by her Guardian Schaffa toward Corepoint, the only city on the far side of the planet where the Gate can be triggered without the central obelisk Essun controls. Essun pursues with a small group, and just before leaving learns she is pregnant by Lerna, her lover and the former healer of Tirimo. The stone eater Hoa transports Essun's party directly through the Earth to intercept Nassun, but a rival faction of stone eaters attacks during the crossing and kills Lerna.

As Nassun and Schaffa travel through an ancient transit system running through the planet's core to reach Corepoint, it becomes clear that the Earth itself is a living, furious consciousness that blames humanity for stealing its moon and enslaving its magic. Interwoven flashbacks reveal that the stone eater Hoa was once the lead "tuner" of the Obelisk Gate in the ancient civilization of Syl Anagist, where tuners—bioengineered from the DNA of an enslaved people—were built to control the Gate's magic. On the eve of the Gate's activation, Hoa and the other tuners discovered that the people whose genes they carried were being tortured and drained as living batteries to power the obelisks. Hoa chose to destroy Syl Anagist rather than let the atrocity continue, but the Earth itself seized the Gate in an attempt to annihilate nearly all life; the tuners narrowly stopped it, at the cost of flinging the Moon out of orbit, becoming the first stone eaters, and triggering the age of Fifth Seasons known as the Shattering. One surviving tuner went on to become the ancestor of all orogenes.

In the present, the Earth strikes back by ripping the magic-conducting iron shard from Schaffa's brain, stripping him of the longevity that has kept him alive. Devastated, Nassun resolves to save Schaffa by using the Gate to turn all of humanity into undying stone eaters rather than destroy the world. Essun arrives at Corepoint and struggles with her daughter for control of the Gate, trying instead to send the Moon back into orbit and end the Seasons forever. Neither can overpower the other, so Essun releases her hold on the Gate to spare Nassun from being destroyed by it—and is turned to stone as a result. Moved by her mother's sacrifice, Nassun abandons her plan to remake or destroy everyone and instead finishes the task of returning the Moon to its proper orbit, ending the Fifth Seasons and beginning the long process of the world's recovery.

In the aftermath, civilization begins to rebuild without the threat of catastrophic Seasons. In a hidden cave, Hoa waits beside a geode until Essun, reborn as a stone eater, emerges from the stone. She wakes with her old resolve intact—to help make the world better—and the two set off together, in a way that suggests Hoa has been the one narrating Essun's entire story to her all along, so that she could remember who she was.