The narrative begins with eighteen year-old Gideon Nav's 87th attempt to escape the Ninth House, a
death cult tasked with guarding a Locked Tomb said to contain the Emperor's greatest foe and by whom Gideon was raised in
indentured servitude. Her plans of fleeing to join the Emperor's armies (called the Cohort) are quickly foiled by her lifelong antagonist and heiress of the Ninth House, Harrowhark "Harrow" Nonagesimus. Despite their clashing personalities and mutual hatred of each other, Gideon is Harrow's only real choice of cavalier, primarily due to a purported atmospheric contamination incident around the time of their births that killed the rest of the Ninth House's children. Harrow offers Gideon a commission in the Cohort if she serves as cavalier during the Emperor's trials. Harrow and Gideon travel to Canaan House, a decaying mansion on the planet of the First House, where they meet the heirs and cavaliers of the other Houses. The group is tasked with exploring the mansion to discover the secrets of Lyctorhood. Harrow initially treats Gideon as a liability and disappears for long periods of time to work on her own, leaving Gideon to wander Canaan House and interact with the other Houses. After one such prolonged absence, Gideon goes looking for her and, with the help of the Sixth House, finds her in a hidden basement containing a plethora of necromantic experiments left by the Emperor and his original group of Lyctors. It becomes apparent that each House's necromancer and cavalier must work together to complete the puzzles in the basement, so Harrow and Gideon begrudgingly ally. They make quick progress in the trial due to Harrow's exceptional skill in necromancy and Gideon's perceptiveness and combat skills. Following a series of suspicious deaths, the Houses assume that one or more of them are hunting the others. It becomes clear that Canaan House is designed to prevent any one of the Nine Houses from succeeding without collaborating with or combating each other. The surviving Houses turn to bribery, blackmail, and unsteady alliances. Gideon and Harrow's own relationship reaches a low point over Gideon's infatuation with the terminally ill heiress of the Seventh House, Dulcinea Septimus; tensions run high after Gideon discovers the severed head of the Seventh House cavalier among Harrow's things. Gideon takes her suspicions (and the head) to the Sixth House, who in turn confront Dulcinea. Dulcinea explains that the Emperor's call had "caught out" her House; lacking options, she had been using necromancy to move her cavalier's body about like a puppet since before their arrival at Canaan House. Harrow and Gideon reconcile. Harrow reveals that the atmospheric contamination incident was an intentional human sacrifice by her family in order to create a necromancer powerful enough to save the Ninth House from economic and political failure: herself. (An infant Gideon somehow survived the sacrifice, leading Harrow's parents and the rest of the Ninth House to treat her coldly out of fear.) Harrow is haunted by the hundreds of innocent deaths involved in her creation. Her guilt led her to breach the Locked Tomb as a young girl, where she fell in love with the immaculate corpse of the beautiful girl entombed within. Harrow's family died by suicide upon Gideon telling them about her heretical trespass; Harrow has been burdened with secretly leading the Ninth House for years. Gideon and Harrow apologise for their treatment of each other. Harrow acknowledges Gideon as her friend, and they pledge a new alliance: "One flesh, one end." The Sixth and the Ninth houses investigate one of the studies in Canaan House. At the same time, the Second House attempts to call the Cohort for backup and intervention, but the Second necromancer is mortally wounded and her cavalier killed by the priests of the First House in the attempt. Meanwhile, the Third House's heiress, Ianthe Tridentarius, deduces how Lyctors are created: a necromancer must extract and devour their cavalier's soul, allowing them to use the soul as a virtually infinite power source and gaining the cavalier's combat skills. Ianthe kills her cavalier without hesitation and becomes a Lyctor, much to the horror of the other Houses. The Eighth House attempts to bring her to justice for the murder of her cavalier, but they are killed when Colum the Eighth is possessed by an angry ghost, who in turn kills his necromancer. After a brief confrontation with Palamedes the Sixth, the Seventh House heiress reveals herself to be an imposter: not Dulcinea the Seventh, but instead Cytherea the First, one of the Emperor's Lyctors. Cytherea explains that she killed the real Dulcinea and her cavalier shortly before arriving at Canaan House and assumed her identity; likewise, she was the one who killed the Fourth and Fifth Houses, in the hopes that doing so would lure the Emperor back to Canaan House. She intends to "kill [him] and burn his Houses" as part of a revenge plot, starting with the remaining heirs and cavaliers. The survivors battle throughout the House, but Cytherea is apparently invincible. Just as Harrow is about to be killed, Gideon intervenes, ending her own life to force Harrow to become a Lyctor. Harrow kills Cytherea before falling unconscious. Harrow wakes up on the Emperor's flagship; she and a wounded Ianthe are the only confirmed survivors of Canaan House. She begs the Emperor to resurrect Gideon, only to learn that Gideon's soul is irreversibly merged with hers. The Emperor reveals that the Empire is in decline and most of the Lyctors have fallen in battle or gone insane. He promises to restore the Ninth House to glory, and in exchange Harrow agrees to serve the Emperor as Harrowhark the First. == Style ==