Seek the Traitors Son by Veronica Roth

Here is a summary with spoilers for Seek the Traitors Son by Veronica Roth

Soldier Elegy Ahn of Cedre and General Rava Vidar of Talusar are summoned separately to the Cenobium, where the augurs deliver a fragmented prophecy. Each is told that one of them will lead their nation to victory over the other, but neither is told which. On top of that, the augers are split as to who the prophecy is about. The prophecy centers on a third person connected to both of them. Elegy is told she will love this person despite already being married to Shir. Both leave the Cenobium uncertain of their futures but aware that their lives are now tied together. Elegy isn’t sure what to think of everything though, she thought her life was perfect, she has both a powerful father and mother, but she was able to stay in the background of their succsess. Meanwhil Genral Rava has a ruthless reputation; it makes Elegy worried that Rava will come to hurt Elegy. Elegy has to go home and tell Shir, her husband about this new person that is going to be involved in their lives. 

The world has been permanently altered by the Fever, a virus that kills, resurrects, and grants supernatural abilities to those who survive. Cedre fears infection and views Fever gifts as dangerous, while Talusar embraces the virus as sacred and builds its society around those powers. Political tensions between the two nations remain high after generations of conflict. Theren Forint, the son of Talusar exiles living in Cedre, is chosen to swear the Knight's Oath. Whle he lives off planet, it is seen as an honor to do so, to bind his life to another, but he isn’t sure what to think about this, knowing it will change his life. Once on earth, he meets Elegy, and learns he is to be bound by the Knights Oath to her, despite his mixed heritage making him an outsider among both nations.

The ceremony happens and Theren becomes Elegy's knight. Almost minutes later, Talusar launches a surprise attack on the sacred Getty (todays modern Getty museum). General Rava Vidar leads the assault. Elegy's mother, the Sword of Cedre, is killed along with Elegy's husband, Shir. The attack devastates Cedre's leadership and forces Elegy to flee, leaving his brand new oath with Elegy behind as well. During the invasion, Theren is unable to stop the attack, and many people view his Talusar ancestry as evidence that he betrayed Cedre. His reputation is permanently damaged despite his loyalty.

As Cedre struggles to recover from losing their leader, both Elegy and Rava attempt to understand more of the prophecy, though they do so at different times. The augurs reveal that the prophecy depends on three people acting together, referred to as the fulcrum. One must bear the Vidar name, one must be connected to the Cenobium, and one must represent both death and love. Neither side fully understands who fulfills each role, but both begin searching for the remaining pieces while preparing for continued war that has now officiay begun.

Theren has been on the run, along with the remaining knights ever since the ceremony attack. Their fleeing takes them into the desert, where they are caught bt some rebels, who Theren is suprised to learn is lead by his mom. They try to get information out of him, and try to get him to join their cuase but he refuses. So in return Theren and all the other remaining knights that were captured are deliberately infected with the Fever. Theren’s mom tells him that he hopes he comes back a second time. Some knights die permanently, while others survive the resurrection process and awaken with supernatural abilities. Theren develops the ability to sense and read emotions. The Fever also restores suppressed memories and exposes long hidden truths about the past. Throughout this ordeal, Theren continues struggling with guilt over the Getty attack and the divided loyalties created by his family's history, all of which he tries to keep hidden. 

A couple of years go by, and evidence begins to emerge that the Fever and parts of human civilization originated from contact with an advanced extraterrestrial civilization. Strange metals, unusual plants, ancient technology, and hidden messages point toward an interstellar origin. A mysterious flowering plant responds only to Theren because of his ancestry. It becomes clear that his parentage connects him to these ancient visitors, making him essential to understanding the technology they left behind.

Elegy eventually becomes a Scout and spends time traveling with her Talusar-born foster sister, Hela. Meanwhile, Theren is captured by Talusar forces and serves as Rava Vidar's truthsayer because of his emotional abilities. Although he appears loyal to Talusar, he secretly works to maintain his independence. 

Elegy has long ago given up hope about finding her knight Theren; she beleived that he died during the attack. One day one of her soldiers tells her that there was a sighting of her, older, but for sure him. Elegy decides to infiltrates Talusar territory using disguises to see if she can find Theren and talk to him. She is captured quickly, and the people are worried because she had a tracker on her, though she is not sure where that came from. She is thrown into the prisons to be interogated, the normal mind reader is not able to work on her because she was trained for that type of torture. So Rava’s truthsayers is brought in, and that is when Elegy sees Theren for the first time in years. Theren works on her to try and get answers but gets nothing. Once eveeryone is gone, Theren comes back for Elegy, and the two reunite before working on escaping together.

As they travel, Elegy and Theren confront the consequences of the Getty attack and the deaths of those they loved. Elegy continues mourning Shir, who also died during the attack,  while gradually developing feelings for Theren, fulfilling part of the prophecy given at the Cenobium. Theren also reveals that he has some memory gaps. Theren also comes to terms with his past, his family's actions, and his place between Cedre and Talusar. Their relationship develops alongside efforts to stop the growing conflict.

The prophecy becomes clearer as the identities of the three central figures emerge. Theren is confirmed as the fulcrum connecting both nations through his ancestry, abilities, and role in the prophecy. Additional revelations about the augurs, the Fever, and ancient alien technology explain that humanity's future depends on recovering forgotten knowledge and activating technology left behind by those who first brought the Fever to the planet.

Talusar eventually unleashes a large scale biological assault using the Fever, causing widespread infection and chaos across Cedre. Elegy and her allies race to stop the outbreak while uncovering the remaining secrets of the prophecy. To gain the foresight necessary to lead her people, Elegy willingly submits to the Fever despite the risk of death. She survives and gains prophetic abilities that allow her to help guide Cedre's future.

By the end of the novel, the major pieces of the prophecy have fallen into place. Theren's extraterrestrial ancestry explains why only he can activate certain ancient technology, including the mysterious flowering plant that sends a signal into space. The augurs' true purpose and the origin of the Fever become clearer, and preparations begin for humanity to travel beyond its home system using ancient technology. Elegy, Theren, and the other individuals connected to the prophecy survive and prepare to face the larger conflict that lies beyond their world.