Apollyon (covenant, #4)

Apollyon opens with Alexandria "Alex" Andros imprisoned in a basement cell at a remote cabin in Apple River, Illinois. Alex is one of two Apollyons, rare warriors who wield control over the four classical elements and a fifth, most powerful element called akasha. Since Awakening into her full power, she has been mentally bonded to Seth, the First Apollyon, whose influence has overridden her will. Bars forged by the god Hephaestus and wards drawn in Titan blood suppress her abilities, while Aiden St. Delphi keeps vigil outside the cell. In this world, pure-bloods form the elite class of demigod society, and Sentinels serve as their warrior guardians. Aiden, a pure-blood Sentinel and the man Alex loves, tries to reach the real Alex beneath the bond.

Through the bond, Seth urges Alex to escape so he can transfer her power to himself and become the God Killer, a being capable of destroying the gods. He promises they will free the enslaved half-blood servants, including Alex's father, Alexander. He also reveals that Alex's stepfather Lucian orchestrated attacks on the Covenants, the society's protected compounds, by feeding captive pure-bloods to daimons as bait. Despite her alignment with Seth, Alex is horrified by the alliance with daimons, monstrous creatures that drain life from their victims. Daimons transformed her mother into one of their kind and killed her best friend Caleb. Alex tells Aiden about the conspiracy, betraying Seth's confidence because the alliance violates something fundamental in her. Caleb then appears as a shade sent by Apollo, the god allied with Alex's side, urging her to break the bond. Alex refuses, clinging to the belief that her power can protect everyone she loves.

Alex uses an Apollyon compulsion, a form of mind control, to manipulate her friend Olivia into helping pry open the cell door, then escapes into the woods. Aiden catches her, and they fight brutally, though Aiden never strikes offensively. Flooded with Seth's encouragement, Alex pins Aiden and summons akasha to kill him. But sunlight catches the crystal rose necklace around her neck, a gift Aiden crafted for her, triggering a flood of genuine memories. The authentic love overpowers the bond. Alex constructs powerful mental shields against Seth, and the cord connecting them snaps back. Freed, she sobs in Aiden's arms, overwhelmed with horror.

The god Thanatos arrives with furies to neutralize Alex, but Apollo intervenes, confirming she has broken the bond. Back at the cabin, Alex apologizes to everyone and learns of the global destruction the gods have unleashed in retaliation: earthquakes, tsunamis, wildfires, and military conflicts. Aiden reveals he banned Apollo from the cabin because Apollo planned to kill Alex if she reconnected with Seth.

Laadan, a pure-blood woman close to Alex's father, shares devastating news: Alexander resisted the Elixir, a substance used to suppress the will of half-blood servants, from the beginning. The Masters, the overseers who control enslaved servants, punished him by cutting out half his tongue. He watched over Alex from afar but could never speak to her. Alex is enraged but exercises restraint, recognizing that the Breed Order, the system of institutionalized half-blood servitude, represents a larger injustice. Her uncle Marcus opens up about Alex's mother, admitting he took out his anger about his sister's forbidden relationship on Alex, and the two reconcile.

While the group trains, Seth reaches Alex through the bond with the help of Hermes, the messenger god, pulling her into a mental confrontation where he warns he will find her regardless of her shields. Alex and Aiden work through a rift caused by Aiden's guilt over previously placing her on the Elixir against her will.

Apollo returns with a talisman that hides Alex's energy from the gods and proposes a mission to the Underworld to find Solaris, the original second Apollyon, who may have known how to prevent the power transfer. Seth contacts Alex through the bond, admitting he cares about her, but he refuses to accept that Lucian is manipulating him.

Alex and Aiden enter the Underworld through a gate at Stull Cemetery in Kansas, fighting past skeleton guards, a three-headed hellhound, and spirits that take the forms of their dead loved ones. They traverse the Asphodel Fields and reach the Plain of Judgment, where Caleb meets them. Persephone, Queen of the Underworld, grants them access to the Calling Waters, a basin that can summon any soul. Solaris reveals that the power transfer can be reversed: Alex must press her flesh to Seth's and invoke each of the four Apollyon marks, symbols tied to her power, by their true names in the correct order. The method is identical to how Seth would drain her, so Alex must complete the ritual first. Solaris warns that becoming the God Killer will warp Alex, and Persephone reveals that Solaris's First is imprisoned in Tartarus, implying Seth would face the same fate.

On the return journey, Alex confronts Apollo, having pieced together the gods' true agenda: They need the God Killer to destroy the rogue god aiding Lucian, since gods cannot kill one another. Apollo confirms this and implies the gods will likely turn on Alex afterward. Alex asks Apollo to protect Aiden if she dies and resolves to keep this knowledge from him.

The group travels to the University in South Dakota, where surviving Sentinels are gathering. Dionysus, the god of wine, reports that one of the chief Olympian gods is shielding Lucian and Seth. En route, Lucian's Sentinels ambush the convoy. Alex uses the air element to flip a Hummer, and the group fights them off. Aiden's younger brother Deacon privately tells Alex that losing her would destroy Aiden, urging her not to accept death. Alex rejects the fatalism she has carried since the Underworld.

At the University, the access road is lined with charred vehicles, the remains of people killed by Hephaestus's automatons, half-bull, half-man machines that have turned against the campus. Alex unleashes akasha to destroy several automatons but exhausts herself. An automaton kills Lea, Alex's former rival and ally, snapping her neck before anyone can react.

Inside, Alex learns that Instructor Romvi has arrived from Deity Island, her home Covenant and training school. Romvi tormented Alex during training, and his presence triggers the Apollyon marks on her skin and a nagging memory from the Underworld, where she noticed trophies in Ares's war room. Alex realizes Romvi is Ares, the God of War, and throws a dagger into his chest. He sheds his disguise, revealing himself as Seth's divine patron and the architect of the conspiracy. Ares kills the guards and the Dean in seconds. Alex blasts Aiden and Marcus to safety and locks the doors. When she refuses to reconnect with Seth, Ares systematically breaks her body. Seth's fury floods through the bond as he feels her suffering. Aiden and Marcus breach the doors, Apollo arrives, and a chaotic battle erupts before Alex loses consciousness.

Alex awakens near Olympus, healed by Apollo's son Asclepius, the god of medicine. Her mother Rachelle appears, summoned through a favor with Hades, and embraces her. Rachelle makes Alex promise to let go of her guilt and whispers, "Don't give up hope, baby. Paradise is waiting for you in the end" (409).

Alex wakes in her dorm room beside Aiden, the bond with Seth still humming quietly. In the mirror, she discovers permanent scars across her face and hands. She experiences a wave of panic, recognizing that Ares shattered something inside her. But she recalls her mother's words and Deacon's plea, and resolves to keep fighting, not because she has accepted death, but because she refuses to let Ares, Seth, or Fate write her ending.