A Court of Silver Flames by Sarah J Maas

Here is a summary with spoilers for  A Court of Silver Flames by Sarah J Maas

Nesta Archeron is living alone in a run-down apartment in Velaris. Since being Made by the Cauldron during the war, Nesta has been unable to cope with what happened to her. She has become increasingly isolated, drinks heavily, spends Feyre's money recklessly, sleeps with strangers, and treats nearly everyone around her with hostility. She is haunted by memories of the Cauldron, the war, her father's death, and the power she took from the Cauldron when she was Made. Feyre, Rhysand, Amren, and Cassian finally confront her about her behavior. Feyre gives Nesta an ultimatum: she can either begin training with Cassian, work in the House of Wind's library, and stop living recklessly, or she will be sent back to the human lands. Nesta refuses to accept the decision, but she knows she has no real alternative because she would be vulnerable and isolated in the human world. Cassian takes her to the House of Wind, where she will train with him every morning and work in the library every afternoon.

Nesta initially refuses to cooperate. She continues drinking and deliberately antagonizes Cassian, but she eventually begins working in the subterranean library beneath the House of Wind. The library is home to priestesses who have survived violence and trauma, and its leader, Clotho, communicates through written notes because she lost her ability to speak. Nesta begins shelving books and performing other tasks for the library. She meets Gwyneth Berdara, a priestess who was previously attacked during the massacre at Sangravah and has isolated herself in the library since then. Nesta and Gwyn gradually become friends. Nesta also begins training with Cassian. At first she is physically weak because of months of drinking, poor nutrition, and inactivity. She struggles with basic exercises, but Cassian recognizes that she has determination even when she refuses to admit it. He gradually introduces more difficult exercises, and Nesta begins developing physical strength.

Cassian eventually brings Nesta to Windhaven, where she meets Emerie, an Illyrian female who runs a weapons shop. Emerie has been mistreated by her family because she is a female Illyrian and has been physically injured in the past, leaving her unable to fly properly. Nesta intervenes when Emerie's cousin Bellius attempts to force her to surrender her shop. Nesta threatens him and makes it clear that she will not allow Emerie to be bullied. Emerie becomes another important friend for Nesta. Nesta decides that the training is doing her good, and it could maybe help others grow as well. She decides to start advertising the sessions at the library. For weeks no one shows up, but eventually Gwyn does. Gwyn later begins training with Nesta, and the two convince Emerie to come as well; and the three women start forming a friendship through their shared desire to become stronger and stop feeling powerless. Gwyn explains that she wants to learn to fight because she never wants to be helpless again. Nesta begins to recognize that she feels the same way.

While Nesta's training progresses, the political situation in Prythian becomes more dangerous. The Night Court learns that the human queens who survived the previous war are still involved in plans against the Fae. One of the queens, Briallyn, has become immortal and is working with the death god Koschei. Briallyn has obtained the Mask, one of the three objects known as the Dread Trove. The other two objects are the Harp and the Crown. The Trove objects were created by the Cauldron and are extremely powerful. Because Nesta was Made by the Cauldron and possesses an unusual connection to its power, she is capable of sensing and finding the Trove. Rhysand, Feyre, Amren, and Cassian want Nesta to help locate the objects before Briallyn can obtain all three. Nesta initially refuses because she hates her Cauldron-given power and does not want to use it.

Nesta eventually agrees to attempt scrying for the Trove. The experience is extremely traumatic because using her power forces her to confront memories of the Cauldron. She struggles to control the visions and becomes increasingly frightened by the extent of the power inside her. During one episode, Nesta's power erupts while she sleeps. Silver flames fill the room, and Rhysand has to intervene to wake her. Rhys realizes that Nesta's power is connected to death and is much more dangerous than anyone previously understood. Nesta continues trying to learn how to control herself rather than simply suppressing everything she feels. Gwyn teaches her a Valkyrie technique called Mind-Stilling, which allows a person to quiet intrusive thoughts and achieve a state of mental control.

Nesta's physical progress becomes increasingly noticeable. She begins training with Gwyn and Emerie, and Cassian starts teaching them more advanced Illyrian combat techniques. Nesta discovers that she enjoys fighting with a sword because it gives her a sense of control. She imagines the people and events that have made her feel powerless while practicing, including the Cauldron, the King of Hybern, the kelpie that nearly killed her, and the poverty and humiliation her family experienced. Gwyn and Emerie become increasingly committed to training with her, and the three women begin referring to themselves as Valkyries after learning about the ancient female warriors who fought alongside Cassian and the Illyrians. Cassian begins teaching them both Illyrian combat methods and the techniques of the Valkyries.

Nesta and Cassian's relationship also becomes increasingly physical and complicated. They begin sleeping together, but Nesta refuses to acknowledge the emotional significance of their relationship or admit that she cares about him. Cassian is already aware that they are mates, but Nesta does not initially accept the bond. Their encounters continue while they train together, and Cassian increasingly realizes that Nesta's hostility is largely a defense against her guilt and trauma. Nesta also becomes jealous and possessive of Cassian even though she refuses to define their relationship. Cassian continues trying to give her reasons to trust him without forcing her to discuss things she is not ready to confront.

Nesta eventually locates the Mask in the Bog of Oorid. She enters the bog and is attacked by a kelpie, a monstrous creature that attempts to drag her underwater. Nesta finds the Mask and puts it on, gaining control over the dead warriors buried in the bog. She commands the dead to attack the kelpie and survives the encounter. The experience demonstrates both the enormous power of the Trove and Nesta's unusual ability to wield it. She returns the Mask to the Night Court rather than keeping it for herself.

The search for the second object leads Nesta and Cassian to the Prison, where the Harp is hidden. There they encounter Lanthys, an ancient death-god who possesses significant magical abilities and attempts to manipulate Nesta. He shows her visions designed to convince her that he can give her everything she wants. Nesta ultimately recognizes the deception and fights him. During the confrontation, she uses a sword she has unknowingly Made with her own power. The sword is later named Ataraxia, meaning a state of inner peace. Nesta kills Lanthys and retrieves the Harp. She also learns more about the Harp's abilities, including the significance of its final string, which can stop time.

During this period, Nesta also begins making significant progress with the 10,000 steps leading down from the House of Wind. At first, she cannot descend very far. Her first serious attempt takes her only 111 steps before her body and mind force her to stop. Instead of simply giving up, she begins using the stairs as another form of training. She repeatedly walks down them after her work and training sessions, gradually increasing the distance she can travel. Eventually she reaches thousands of steps. The stairs become a concrete measure of her physical recovery because she is slowly rebuilding the strength she lost during her period of self-destruction.

After Nesta and Cassian return from one of their missions, Nesta's relationship with the Inner Circle deteriorates further. Feyre has agreed that Nesta should finally be told that the weapons Nesta Made from the Cauldron have special powers, but Nesta discovers that Rhysand, Feyre, Amren, and the others had previously discussed what to tell her without including her in the decision. Nesta is furious that they kept information about her own abilities from her. At the same time, she discovers that Feyre is pregnant. Feyre and Rhys are excited about the pregnancy, but Rhysand and the Inner Circle know something that Feyre does not: the baby has Illyrian wings, and Feyre's body cannot safely deliver an Illyrian child because she does not have an Illyrian pelvis. The pregnancy could therefore kill Feyre and the baby. Rhys has ordered everyone to keep this information from Feyre because he believes that telling her would only frighten her while they search for another solution.

Nesta is devastated and furious when she learns that Feyre has been denied the truth about her own pregnancy. Her anger toward the Inner Circle, particularly Rhysand and Amren, explodes. She manages to conquere the 10,000 steps and goes straight to confront Amren, and the argument becomes physical. Nesta and Amren fight, with Nesta's anger overwhelming their previous friendship. Feyre arrives and attempts to intervene, but Nesta lashes out at her as well. In the middle of the confrontation, Nesta deliberately tells Feyre that her pregnancy could kill her. Feyre is shocked to discover that everyone around her knew about the danger while she did not. Rhysand becomes furious with Nesta for revealing the secret, and the confrontation causes Nesta's relationship with the Inner Circle to collapse. Rhys orders Cassian to take Nesta away from Velaris.

Cassian takes Nesta deep into the mountains rather than returning her to the House of Wind. He brings her on a long hike that he knows will take them several days to reach a remote lake. Nesta initially assumes the trip is another punishment, but Cassian forces her to continue walking rather than allowing her to retreat into her usual habits. The two don’t talk, he just forces her to walk. The physical exhaustion removes many of the defenses she normally uses to avoid confronting her feelings. As they travel, Nesta becomes increasingly overwhelmed by everything she has been suppressing. She is forced to confront her guilt over her father's death, her anger toward Feyre and Elain, her shame about what happened after the war, her fear of the Cauldron, and her belief that she is fundamentally unworthy of being loved. Cassian does not try to fix the situation for her. He stays with her while she finally breaks down and admits how much pain she has been carrying. The two also finally confront the reality of their relationship and the mating bond between them.

After the hike, Nesta returns to the House of Wind changed. She resumes training with Gwyn and Emerie and begins rebuilding her relationships and even apologizng. Her training becomes increasingly serious, and the three women work toward becoming genuine Valkyrie warriors. Nesta also becomes more comfortable using a sword and begins learning how to use the weapons she created through her Cauldron power. Cassian continues training them while also helping them prepare for the Blood Rite, the brutal Illyrian competition in which warriors must survive in the wilderness and reach the summit of Ramiel.

Nesta and Cassian eventually fully acknowledge their mating bond. They continue their sexual relationship, but their connection becomes more emotional as Nesta begins allowing Cassian to see her vulnerabilities. Nesta still won’t fully accept though. Nesta also develops a stronger relationship with the House of Wind itself. She realizes that the House responds to her needs and eventually discovers that the House was effectively created for her through the force of her need for companionship. The House provides her with comfort and companionship when she is alone. Nesta's friendships with Gwyn and Emerie become equally important, and the three women continue training together as Valkyries.

Nesta's relationships with her sisters also begin improving. Elain is more accepting of Nesta's attempts to change, and Feyre eventually begins repairing her relationship with Nesta after the pregnancy confrontation. Nesta still struggles with guilt and anger, but she no longer uses alcohol and sex as her primary methods of escaping her emotions. At the Winter Solstice celebration, Nesta attends the gathering with the Inner Circle and is more comfortable around them than she was at the beginning of the book. Cassian gives her a Symphonia that contains recorded music he collected for her. Nesta also receives a small light from Azriel that she can use for reading. She is able to enjoy the evening without drinking and increasingly feels that she belongs with the people around her.

Briallyn's plans eventually become more direct. She has been using the Crown to control people and has been gathering information about Nesta and the Dread Trove. Eris continues secretly providing information to Cassian and Azriel about Briallyn and Beron's activities. 

Meanwhile, Nesta, Gwyn, and Emerie continue training and become capable enough to attempt the Blood Rite. During the qualifying test, the three women demonstrate that they can fight and function as a team. Cassian is proud of their progress but remains concerned about how dangerous the actual Blood Rite will be.

The Blood Rite begins when Nesta, Gwyn, and Emerie are kidnapped and magically transported into the Illyrian territory where the Rite takes place. The three women are separated when they arrive. Nesta wakes among male Illyrian warriors and realizes that the rules of the Blood Rite normally prohibit the use of weapons and magic. She manages to obtain weapons and begins searching for Gwyn and Emerie. Outside the Rite, Cassian, Rhysand, Azriel, and Feyre realize what has happened, but they cannot enter the Rite to rescue them. Anyone who interferes with the Blood Rite is subject to execution, so Cassian is forced to remain outside despite knowing Nesta is in danger.

Nesta eventually finds Emerie, who has been injured, and helps her survive. She then searches for Gwyn. Gwyn has also been fighting to survive and is eventually found by Nesta. The three women reunite and decide that they will reach the summit of Ramiel together. They survive attacks from Illyrian warriors, gather food and weapons, and use their Valkyrie training to defend themselves. Rather than abandoning one another to improve their individual chances of winning, they remain together. Their objective becomes reaching the summit as a group.

As they approach Ramiel, the three women are pursued by a large group of Illyrian warriors. Emerie is injured, and Gwyn becomes exhausted. Nesta realizes that they cannot all reach the summit if they continue running together because the pursuing males will catch them. She chooses to stay behind at the Pass of Enalius and hold off the attackers while Gwyn and Emerie continue toward Ramiel. Nesta fights alone, knowing that she may die, and uses the same basic strategy associated with an ancient warrior who once defended the pass. Gwyn and Emerie eventually reach the summit and touch the stone at the top of Ramiel, becoming Carynthians. Nesta survives long enough to reach them and completes the journey as well. The three women become the first female warriors to achieve the highest rank of the Blood Rite.

At the summit, Nesta discovers that Cassian has also been brought into the conflict. Briallyn has used the Crown to control him. Cassian is forced to fight Nesta while under Briallyn's control, and Briallyn orders him to kill himself when he cannot defeat Nesta. Nesta uses her Cauldron power to destroy Briallyn rather than allowing Cassian to die. Her power effectively Unmakes Briallyn, destroying the queen and breaking her control over Cassian. Nesta is then left with the remaining Trove objects and the knowledge that the Cauldron power inside her is still capable of enormous destruction.

The danger is not over because Feyre has gone into labor prematurely. Her pregnancy has reached the point where the baby must be delivered, but Feyre's body cannot safely give birth to an Illyrian-winged child. Feyre, Rhysand, and the baby are all in danger of dying. It is at this point Rhysand tells everyone a secret, he and Feyre bound their lives together; if she dies he dies, and vise versa. Nesta realizes that the three objects of the Dread Trove can give her access to an extraordinary level of power. She realizes this as Mor is holding a baby that isn’t moving and Feyre is bleeding out. She uses the Mask, Crown, and Harp together. The Harp allows her to manipulate time, while the Mask connects her to death, and the Crown controls minds. Nesta uses the combined power to stop time and prevent death from taking Feyre and the baby.

Nesta then makes a bargain with the Cauldron. She offers to return the majority of the power she stole from it when she was Made in exchange for Feyre's life and the life of Feyre's child. Nesta's power is consumed by the Cauldron, and she gives up most of what she gained from it. Before the exchange is complete, Nesta also uses her remaining power to alter Feyre's body so that she can safely carry and deliver future Illyrian children (she doesn the same for herself). Feyre, Rhysand, and the baby survive. Nesta retains a smaller portion of her power, but she has sacrificed the overwhelming majority of it to save her sister and nephew.

Afterward, Rhysand acknowledges what Nesta has done for Feyre and the baby. Nesta's sacrifice changes her relationship with the Inner Circle because she has demonstrated that, despite everything that happened between them, she was willing to give up her own power to save Feyre. Nesta and Cassian remain together and accept their mating bond. Nesta continues training with Gwyn and Emerie, and the three remain Valkyries, they even start teaching other priestesses in the library. Cassian and Nesta begin preparing for their future together, while Nesta also accepts the House of Wind as her home. She visits her father's grave with Feyre and Elain, confronting the complicated feelings she has carried about his death and their family.