Saeris awakens on the tomb floor in Ammontraíeth, confused and injured, as a figure emerges from the active quicksilver pool. She recognizes Orious, seneschal to King Belikon, wearing a chain with a golden orb. Five guards in green velvet follow, arrows nocked. Orious presents Belikon’s offer: sanctuary from spreading black rot in exchange for Saeris becoming his Alchemist, returning Everlayne (his daughter and Kingfisher’s half-sister), delivering Kingfisher for execution, and swearing an oath on the Firinn Stone. He reveals Kingfisher, Renfis, and Lorreth are oath-sworn to Belikon and demands the return of Solace, Saeris’s sword. When Saeris refuses, magically splitting her sword into two short swords, Orious orders her killed. Saeris deflects their arrows with her magic. The guards draw null blades—weapons that resist her power and cause nausea. During the brutal fight, Saeris kills one guard. Kingfisher arrives through the pool with Carrion, who carries an unconscious Hayden. Before she can respond, Kingfisher kills one of the guards with his shadow magic. Though Kingfisher offers help, Saeris insists on finishing the battle alone. She merges her twin swords back into Solace, projects a massive magical shield from her hand, and kills the remaining guards with an energy blast. Kingfisher pins Orious with shadows. Saeris stops him from torturing the seneschal, choosing instead to send him back as a message. Kingfisher forces Orious to crawl into the pool. Saeris closes the gate easily, feeling that her bond with the quicksilver has strengthened, then embraces Kingfisher. Carrion presents the unconscious Hayden.

At Cahlish, Hayden remains unconscious following quicksilver travel. Healer Te Léna explains humans recover more slowly than Fae and reveals her magic cannot heal Saeris’s wounds, which are mending naturally. In Kingfisher’s chambers, fire sprite Archer fusses anxiously over unfinished preparations. He explains a ceramic kingfisher ornament inspired Kingfisher’s name—Lady Edina bought it while pregnant and treasured it. After bathing, Saeris greets Kingfisher, who empties sand from his boots. He admires her twin short swords, reading their Old Fae names: Erromar means Mercy, Selanir means Honor. He reports finding Lorreth and arranging for their horses to cross the river, noting that the rot has destroyed the Fae camp. Saeris worries about the oath to Belikon. Kingfisher reassures her that Belikon’s compulsion cannot reach them within Cahlish or Ammontraíeth’s wards unless Belikon is physically present. Saeris notices silver veins beneath her skin and quicksilver runes now marked permanently on Kingfisher’s hand. He professes his love, declaring she is his strength. They sleep and later make love. During climax, ink from Saeris’s God Bindings—the markings that wind around her wrist and arm—transfers to Kingfisher’s chest below his collarbone, though he does not notice.

Archer wakes them urgently—Te Léna needs help with Everlayne. In Everlayne’s room, they find her in violent seizure. Carrion explains she attacked and strangled Te Léna. The healer describes an unbreakable wall around Everlayne’s consciousness. A shadowy figure appears in Everlayne’s eyes. At Saeris’s touch, a voice speaks through Everlayne, declaring the gate is open and cannot be closed. Saeris senses dark oppression, and the voice stops. Everlayne falls still. Distraught, Kingfisher leaves. In the dining room, he orders abandoning the border, sends Lorreth to fetch Iseabail, and plans to return to Ammontraíeth for books. At the Blood Court, they find the tomb door barred. Kingfisher smashes through and intimidates guards Anterrin and Khol. He will take Onyx to Saeris’s chambers while she and Carrion seek Taladaius. High bloods kneel as they pass. Saeris reveals her connection to sire Taladaius feels like a “leash.” Outside Tal’s chambers, they confront Zovena, Keeper of Missives. After tense exchanges where Zovena threatens Carrion and taunts Saeris as King Killer, Saeris uses royal authority to command Zovena’s silence and forces her to deliver intercepted letters meant for the shunned Foley. Tal opens his door, naked and bloodied, interrupting them.

Kingfisher brings Onyx to Saeris’s chambers, singing a Fae lullaby. He summons smoked trout for the fox using his magic. As he leaves, Onyx gifts him a perfect pine cone. In the library, Guru the shadow cat takes an interest in Kingfisher and leads him through the stacks to a hidden sanctuary. Guru guides him to a roof access before merging into his shadow. On the roof, Kingfisher finds Foley writing at a desk—their first meeting in centuries. Foley refuses to leave Ammontraíeth, breaking down as he describes his vampiric hunger and shame. He reveals he has not fed on Fae blood since Ajun, nearly a millennium ago, surviving on rats and birds. To prove Foley has control, Kingfisher cuts himself. Foley recoils in horror while Guru drinks the blood. Kingfisher distracts Foley with a wooden box containing Joshin, the scorpion demon he bargained with, explaining that the loopholes in his deal allow him to keep it contained indefinitely. He asks Foley, as his brother and a member of the Lupo Proelia, their warrior brotherhood, to safeguard the box. Foley agrees and asks to hear the demon’s secret.

Saeris and Carrion enter Tal’s chambers, discovering seven naked Fae “females” in his bed. When Carrion flirts with Tal and asks to join, Tal playfully challenges Carrion’s ability to satisfy multiple partners; Carrion backs down. Tal explains he barred the tomb against Belikon’s forces and gives Saeris a null blade, warning that it made him feel inclined toward self-harm. Yanica, the royal master tailor, arrives for measurements. Tal informs Saeris that the Evenlight Ball is mandatory—she must preside and oversee the selection of a new Lord of Midnight. Grudgingly, she agrees and allows measurements. In the library, the null blade causes numbness and aching runes. Hundreds of paper stargazer birds swarm around her in a vortex. One deliberately cuts her cheek for blood. The birds fall, unfolding into pages containing alchemical text. Saeris realizes the stargazers were a hidden book all along. The pages reassemble into a navy volume with a silver butterfly. Inside, a note from Edina and Saeris’s blood drop confirms that the book tested her identity. Despite Edina’s earlier warning to keep it secret, Saeris shows Kingfisher. He reads the first page, then returns it, saying it is meant for her alone, and he trusts his mother’s wishes.

A letter within the book, addressed to Kingfisher from his mother, Edina, is revealed. She explains that Saeris's choosing to show him the book represents a pivotal decision, strengthening their bond. Edina tells him her lingering as a spirit was necessary and that she cherished watching him grow from beyond death. She states she has no regrets, expresses her love and pride, and instructs him to return the book to Saeris.

Saeris reflects on passing Edina’s test of character by showing Kingfisher the book. Frustrated by constant ball-planning interruptions, she delegates everything to Tal and returns to Cahlish. At the forge, the null blade resists all magical manipulation and makes her nauseous. Kingfisher’s shadow magic also fails against it. Saeris confronts Carrion about claiming his throne, as he does not seem interested in it. Fire sprite Archer overhears Carrion saying that no one would honor his claim. Archer tells Carrion that the Yvelian people have prayed for the Daianthus heir’s return for generations. Carrion is left speechless. Archer runs outside to pick the flowers Edina loved. A pillar of flame erupts. Saeris grabs the null blade and finds an infected feeder attacking Archer. She throws the blade at it, but the weapon is absorbed, making the feeder stronger. The feeder bats Carrion into a wall; behind him, the vines there briefly bloom with white flowers and immediately wither. When the feeder springs again, Archer throws himself at it to protect Saeris. The feeder bites Archer’s neck, releasing glowing magma-like liquid. When the liquid, soon to be revealed as brimstone, touches the infected creature, the rot inside it blisters, splits open, and boils away, leaving the feeder destroyed. Archer lies dying, his flames extinguished. Suffering burns, Saeris and Carrion carry the heavy, cooling sprite inside.

In the scorching fire sprite quarters beneath Cahlish, Lorreth explains that brimstone is the sprites’ life-element. He confirms it destroyed the infected feeder and rot. Iseabail reveals brimstone is finite and non-regenerating—Archer survives only because his pyre, or group of fire sprites, donated portions of their own. Saeris and Kingfisher realize that obtaining enough brimstone to fight the rot would require exterminating all fire sprites. Near dawn, Kingfisher takes Saeris to the huntsman’s cottage for peace. He tells her how his father sought a love potion from a creature called a graven to woo Edina. His father refused to trade his foot or all his shadows, finally offering one shadow. The graven, feeling cheated when sunlight revealed its new shadow at midday, chased his father with a scythe and severed his toe. The potion proved worthless. Kingfisher, whose stories of his father came from Archer, promises to support Saeris without controlling her. When she asks if he will carry her when overwhelmed, he says it would be his honor. She vows to become his shadow; he replies that his strength has always been his shadows.