The Stone Sky recap

Essun wakes in Castrima with a stone arm, Alabaster dead, and a comm that must abandon its geode and walk north through the ash to the emptied city of Rennanis. Nassun and Schaffa travel to Corepoint on the far side of the world, where the obelisks can be commanded directly, and where Nassun intends to open the Obelisk Gate and bring the Moon down to end all life. Between them, Hoa tells his own origin: forty thousand years ago he was a tuner, an engineered being built by the civilization of Syl Anagist to run a planetary magic engine, and that civilization's attempt to tap the Earth's core cost it the Moon and made the planet an enemy of everything living. Essun has Hoa carry her through the rock to Corepoint, arriving as Nassun takes hold of the Gate. Rather than fight her daughter, Essun opens the Gate and catches the Moon, returning it to its orbit and ending the Seasons for good. The effort turns her entirely to stone. Nassun completes the work, chooses to let the world live, and Hoa remakes Essun as a stone eater.

Previously, in The Fifth Season and The Obelisk Gate. On the continent called the Stillness, civilizations end regularly in seismic catastrophe. Orogenes can control the movement of the earth; they are hated and hunted, and the empire of Sanze controlled the ones it caught through the Fulcrum and through the Guardians, who can shut an orogene's power off by touch. Damaya, Syenite and Essun are one woman at three ages: a girl given up by her family to a Guardian named Schaffa, a Fulcrum orogene sent on assignment with the ten-ringer Alabaster, and a middle-aged woman hiding in the small comm of Tirimo. As Syenite she escaped to the free island comm of Meov, had a son by Alabaster, and killed that son with her own hands when Guardians came for him, destroying Meov with an obelisk in the process. Alabaster later tore a rift across the continent at Yumenes, beginning a Season expected to last centuries. The same day, Essun's husband Jija beat their three-year-old son to death for being an orogene and fled south with their daughter Nassun. Essun went after them, gathering Hoa, a stone eater in the shape of a boy, and Tonkee, a geomest, and reached Castrima, an underground comm inside a geode led openly by an orogene named Ykka. Alabaster was there, dying and turning to stone, and he explained what he had done: the Seasons are the consequence of an ancient theft, the Moon was flung out of its orbit tens of thousands of years ago, and catching it and returning it would end the Seasons forever. Doing so requires opening the Obelisk Gate, and Essun was the only candidate he had. Jija took Nassun south to Found Moon, a school for orogene children run by Schaffa, who had survived Meov with his memory gone and a corestone burning in his head. Nassun grew more powerful than anyone there and loved Schaffa as she had never loved her mother. When the compound's Guardians turned on them and Jija came for her with a knife, she turned her father to stone. Alabaster died. The army of Rennanis besieged Castrima, and Essun opened the Obelisk Gate, turned the entire army to stone, and lost her right arm to the same process. Mother and daughter are now the only two people alive who can open the Gate: Essun to catch the Moon, Nassun to end the world with it.

Essun wakes in Castrima weeks after the battle. Her right arm is stone from the shoulder down, Alabaster is dead, and Lerna tells her two things she has not accounted for: that the Gate will take the rest of her the same way if she keeps using it, and that she is pregnant. Castrima cannot stay. The geode's machinery is failing and its stores will not carry the comm through the Season, but the army Essun turned to stone came from Rennanis, a whole city in the north that is now standing empty and provisioned. Ykka's decision is to walk there: hundreds of people, through ashfall, across a continent, in winter. Far to the south, Nassun leaves the ruins of Found Moon with Schaffa. Steel has told her where the obelisks can be commanded directly, a place called Corepoint on the other side of the world, and she means to reach it. Schaffa is deteriorating: the corestone that keeps him alive is also devouring him, and the pain drives him toward harming the child he is protecting. Nassun holds him together with the silver, which is not something a Guardian should permit and not something a child should be able to do.

Hoa begins telling his own beginning. Forty thousand years ago he was called Houwha, one of six tuners built by the civilization of Syl Anagist: engineered beings, grown rather than born, whose purpose is to run the Plutonic Engine, the vast machine whose components are the obelisks. Syl Anagist runs on magic drawn from living things, its buildings are grown, and its great project, Geoarcanity, will tap the planet's core for limitless power. The tuners live in a facility, are studied, tested and maintained, and are not considered people by anyone who works on them. Into that arrangement comes Kelenli, brought in as their instructor: like them, but able to speak aloud and to walk the city as a citizen. Castrima's people leave the geode and start north. The march is what an ash winter does to a large group of civilians: bad water, failing feet, ash lung, and comms along the route that will not open their gates. Ykka drives them forward. Essun keeps the road passable with orogeny and pays for it, and Nassun, on the other side of the world, walks south with a Guardian who is dying by degrees.

Kelenli takes the tuners out of their facility and walks them through Syl Anagist, which they have never seen. They learn what the city is built on. The people its founders conquered, the Niess, were declared not-quite-human and destroyed, and the tuners were built in their image to be tools. They see the briar patch, where human beings are kept alive, wired into the network, as raw fuel for the magic the city runs on. Kelenli is one of them, made closer to human and permitted to move freely, and she has brought them out deliberately to show them the truth before the engine is launched. Castrima's march north turns worse. A pack of kirkhusa, once domestic animals, has gone carnivorous and hits the column; people are killed and maimed. Maxixe, an orogene Essun trained beside at the Fulcrum decades ago and who has lost his legs since, holds part of the line. Danel, the captured Rennanis general, writes down what happens because someone should. Lerna takes Essun's stone arm off, and Essun keeps walking.

The tuners understand what they are for and what will happen to them afterward: they are components, and components are decommissioned. Kelenli's teaching has given them the last thing the engine needed and also the will to use it differently. Gallat, the conductor who has raised and managed them, treats the discovery as a maintenance problem. Nassun and Schaffa reach an ancient deadciv station in the far south and find the transport still working: a vehicle that travels through the body of the planet itself. It carries them under the world to Corepoint, a city built by that same lost civilization on the far side of the ocean, standing over the shaft its builders drilled toward the Earth's core. Corepoint is intact, empty of people, and inhabited by stone eaters who watch and say nothing. Schaffa is barely conscious by the time they arrive. Castrima's column reaches the north with fewer people than set out, and finds Rennanis exactly as the Gate left it: a walled, warm, provisioned city full of statues that were people.

Castrima takes possession of Rennanis. There is food, shelter and a functioning greenland, and there is also the problem of living among the stone bodies of the army Essun killed. Ykka begins the work of making a city out of a refugee column. Essun, who is running out of body and time, decides she cannot stay: Alabaster's work is unfinished, her daughter is at the far end of the world, and the only way to reach either is through the rock. At Corepoint, Steel shows Nassun what the place is for. The obelisks answer here more readily than anywhere on the Stillness, and the Gate can be opened from the platform above the shaft. He tells her plainly what she can do with it, including the option of bringing the Moon down and ending everything. Nassun is not persuaded by him so much as already decided. Her one remaining tie is Schaffa, whose corestone will not let him die and will not let him rest, and she is prepared to burn the world to end his pain and her own.

In Syl Anagist, the launch approaches and the tuners make their choice: rather than let the engine simply take what the city wants, they intend to turn it. Kelenli has told them what the conquered Niess were and what their own existence is worth to the people who built them, and the six of them agree that the machine will not be used as designed. Essun asks Hoa to take her to Corepoint. He carries her through solid rock, a journey of days in which she is held inside stone and cannot move, breathe or die, and he tells her the rest of his story on the way. She arrives on the far side of the world stone-armed, weak and out of time, and she knows what she will find: her daughter, at the Gate, intending to end the world. At Corepoint, Nassun has reached the same point from the other direction. Schaffa is failing, the obelisks are gathering overhead, and she is ready.

Syl Anagist's engine launches and the Earth answers. What the tuners set in motion, and what the planet does with it, ends the civilization that built the obelisks: the Moon is thrown out of its orbit, Syl Anagist is destroyed, the Earth becomes the enemy of everything living, and the tuners are turned to stone, the first of the stone eaters. That is where Hoa comes from, and why he has spent forty thousand years waiting for an orogene who could put the Moon back. At Corepoint, Essun and Nassun stand at the Gate together. They do not fight. Nassun sees that her mother crossed the world to reach her rather than to stop her, and Essun opens the Gate and does what Alabaster asked: she catches the Moon and returns it to its orbit. The effort completes what the Gate began in her, and she turns entirely to stone and dies. Nassun carries the work through, chooses to let the world live, and releases Schaffa at last, ending his pain. With the Moon restored, the Earth's grievance is answered and the Seasons will end. Hoa remakes Essun as a stone eater. She wakes into that existence with his whole account already told to her, and Nassun, alive, is carried north to Rennanis, where Castrima's people take her in.

Essun reaches Corepoint, carried through the rock by Hoa, arriving stone-armed, dying and out of time. Nassun has already taken hold of the Obelisk Gate and means to bring the Moon down and end everything. They do not fight over it. Essun opens the Gate too, and instead of turning it against her daughter she uses it for what Alabaster wanted: she catches the Moon and pushes it back onto its orbit. The effort finishes what the Gate started on her body. She turns entirely to stone and dies. Nassun, holding what is left of her mother, carries the work through to the end and chooses not to destroy the world after all. Schaffa's long torment ends at Corepoint; Nassun releases him and he is finally at rest. With the Moon returned, the Earth's grievance is answered: the Seasons will wind down, the ash will clear in time, and the Stillness becomes survivable in a way it has not been for forty thousand years. Hoa remakes Essun as a stone eater, and the book closes with her waking into that new existence, with Hoa beside her and the whole of his account already given. Nassun survives and is carried north, where the people of Castrima, now settled in the emptied city of Rennanis, take her in. Ykka, Lerna, Tonkee, Danel and the rest inherit a world that has a future.