In a post-apocalyptic world where mankind has battled a genetically altered sect for centuries, sixteen-year-old Ruin Gadiel carries an extraordinary burden. In a world of over ten billion people, he carries within him the last soul—he just doesn't know it yet.

When his home is violently overrun by fallen angels and demons crossing the veil separating their realms, a merciless fallen angel executes Ruin's family with the very blade Ruin had forged with his father, a sword with an opal-embedded hilt.

Ruin is rescued by the First Order, a group of angels led by the formidable Michael. Together, they uncover a shocking truth: Ruin carries the last human soul, a prize that Abaddon seeks to claim. The Creator reveals that humanity's reliance on technological reincarnation has drained their souls, leaving a soulless population birthing soulless children. Over centuries, this process has eradicated the souls of mankind. Prophecy warns that when the Well of Souls runs dry, Armageddon begins, marking the rise of the Black Seraph—a figure whose choice will determine the fate of all.

The fallen angel Abaddon unleashes his forces to hunt Ruin and uncovers a prophecy revealing that the one whose blade kills Michael will become the Black Seraph. Determined to control the Black Seraph and the fate of Armageddon, Abaddon plots Michael’s death.

Ruin and the angels encounter Kedesh, a genetically altered girl with a deadly poisonous touch. Though their worlds are at odds—Ruin’s father spent his life battling her kind—a forbidden romance starts to bloom between them. Alongside Thalismar, a rebellious water angel with the power to summon tornadoes and transform rain into icy bullets, they confront escalating dangers as Abaddon’s forces advance.

In the climactic battle, Ruin kills a fire demon and unexpectedly gains the ability to control fire, fulfilling a prophecy that the one with the last soul will acquire power during the end of days. Ruin can now emit streams of fire from his hands. Riding Gabriel, a scout angel, he decimates thousands of demons in Old Manhattan. However, another prophecy unfolds when Michael is slain with the opal-hilted sword Ruin forged. To Abaddon’s surprise, the blade’s owner, not its wielder, becomes the Black Seraph. Ruin ascends as the Bringer of Armageddon— the Black Seraph.

Ruin is transported to a void between realms where he sees the vision of a prophetic tree symbolizing the paths he must choose as the Black Seraph. Guided by a mysterious voice in the void, he is told his decision will determine which side triumphs in the final battle. Remembering Michael’s words, that prophecy is a tree, and sometimes you have to break a few branches— Ruin makes a different choice. He’s not going to break any branches. He’s going to burn the whole tree down, defy prophecy, and forge his own destiny. Returning to his world, he harnesses the Black Seraph's power, banishing the fallen angels, demons, and all celestial beings back through the veil, restoring his world's former reality, but at a great cost—the cost of his own life.

Ruin's sacrifice and the struggle against their shared enemy, Abaddon's army, have united humanity—bringing peace back to his world. As Kedesh copes with Ruin's death and the banishment of her angelic allies, she discovers that Ruin's father's soldiers reincarnated his mother and sister. Though alive, they are without their souls, and his father’s fate remains unknown as his body was never found.

Kedesh returns to Ruin’s family cabin in the mountains with his mother and sister, mourning the loss of Ruin and his father. In the solitude of their home, they begin to heal. One morning, as Kedesh steps outside, she notices a figure approaching from the hill. Her breath catches, and she runs to him—it’s Ruin. Though he sacrificed himself, the Creator has sent him back, stating that the world will need a leader for the challenges ahead.

In the end, the reader learns the identity of the voice in the void—a celestial being called the Storyteller, whose purpose is to chronicle the stories of all worlds across the cosmos. She reveals that Kedesh plays a far greater role in Ruin’s story than anyone had realized. Kedesh is to become the Last Soulurgist, a being capable of transporting souls between realms and creating new ones. Her power could be the key to restoring souls to Ruin’s family and their world. The book, Ruin and the Black Seraph, is revealed to be written by the voice in the void, who, in the final scene, begins writing a new story: Ruin and the Last Soulurgist.