Sweetest Fiend by Raven Kennedy

Here is a summary with spoilers for Sweetest Fiend by Raven Kennedy

Thirty years before the main story,  the continent of Brekon is torn apart by a catastrophic event known as the Break. Varrik is a soldier searching for his younger brother, Trigg, when the earth splits open and molten lava floods the land. Varrik watches as the destruction separates them, then searches through the devastation, pulling burned bodies from the wreckage and hoping to find his brother alive. He eventually discovers evidence that Trigg has died. Before the Break, the Fates had poured too much magic into the world's core, and the resulting disaster was believed to be part of a process that would eventually make the world stronger. Millions of fae die, but the few who survive direct contact with the molten core gain extraordinary magical abilities and become known as Core fae. Nine monstrous beings called Banes also emerge from the fissure. The Core fae spend five years fighting them, eventually defeating them, but each Bane releases a Plague before its death. The Banes also leave behind children who are half-fae and half-Bane. These children become known as fiends and are blamed for the destruction and Plagues that follow.

In Hamelyn, Estinya Zeta, known as Esti, lives with her mother, Marryn, and her aunt, Lenna. Esti is a fiend who conceals her small horns because people believe fiends are responsible for the Plagues. She also possesses an unusual ability to read people's character through the constellations she sees associated with them. She can identify qualities and truths about people, although she keeps this ability private. Hamelyn is currently suffering from the Fly Plague, which has lasted for months. The flies attack any exposed skin, forcing residents to cover themselves and change their daily routines. Esti is already struggling to survive in a community where religious fanatics tell her to repent and other residents target her simply because of what she is. Her mother is also seriously ill, suffering from painful seizures and requiring medicine to manage her condition. Esti and Lenna do what they can to care for Marryn while trying to survive the increasingly dangerous conditions in their city.

One night, Marryn goes outside without covering her skin, and the flies have suddenly died. Esti realizes that the end of one Plague means another has arrived. The Poison Plague contaminates the water, and the city's authorities respond by restricting access to the remaining clean supplies. Esti and Lenna go to the home of the city's leader, Lord Havlon, hoping to obtain water, but learn that the authorities are damming and clearing the lake and will charge residents for what remains. Water becomes scarce and expensive, and the two women are forced to pay several times the expected price for a small supply. On their way home, they are targeted by people trying to steal their water. Esti's former fiancé, Theo, intervenes with his companions and helps them escape. Theo has moved on and is engaged to Gisela, a fae woman whom Esti recognizes as genuinely kind through her ability to read people's character. Seeing Theo again hurts, but Esti has little time to dwell on their past because her mother's health is worsening. When Marryn suffers repeated seizures, Esti realizes they have run out of the medicine she needs. She leaves after curfew to find an apothecary, knowing the risk but unwilling to let her mother die without trying to help her.

Esti obtains medicine but is caught by a patrolman after curfew. As punishment, she is taken to a public platform and brutally whipped across her back with an iron-spiked whip treated with poison. She is left bleeding and unable to escape when the Rat Plague begins. Enormous numbers of rats flood Hamelyn, attacking and devouring people throughout the streets. Esti is still trapped on the platform and struggles to survive above the swarm. The city is overwhelmed until a mysterious figure arrives playing an enchanted pipe. This is Varrik, now known as the Pied Piper, whose music can control the rats. He leads them away and eliminates the immediate threat. Varrik then demands payment from Lord Havlon for his services. Rather than accept money, he demands 130 citizens. Havlon agrees, and Varrik uses his magic to compel the chosen people to follow him. Esti is among them. When she looks at Varrik, she reads three words associated with him: Defender. Thief. Liar. She is taken from Hamelyn alongside other captives, including Theo and numerous fiends, leaving her mother and aunt behind and not knowing where she is at.

Varrik transports the captives toward his territory and eventually takes them to Penbroge Castle. On the way there Esti tries to escape when she sees an opening. The Piper is the one who stops her and forces her to come back to his tent and basically become a prisoner to him. She stays the night in his tent, and the next morning she assumes it will be fine, but instead she is forced to be on his horse; Varrik doesn’t trust her.  When they arrive at the castle, the captives are separated, with the fiends housed apart from the other fae. Esti collapses because the infection in her back has become severe. Varrik carries her to his own room and summons a healer, but the treatment is so painful that Esti believes she may die. Varrik intervenes, dismisses the healer, and takes over her care. He remains beside her while she is unconscious and continues treating her wounds. When Esti recovers, she learns that he stayed with her throughout her illness. Although she is still his prisoner, his concern for her complicates her understanding of him. She continues trying to learn why he took her and the other fiends, while Varrik repeatedly avoids giving her a complete explanation.

For the next few days Esti is trapped in his room with nothing to do. She passes time by bantering with him, and even pulling little pranks on him. Though one day, he ends up leaving. Esti discovers that Varrik has brought other fiends to his territory, and she begins interacting with them. She learns that the people taken from other communities have often been rescued from anti-fiend persecution and that their experiences differ from what she endured in Hamelyn. Varrik eventually takes the fiends to a settlement where Ciona, a Core fae with the ability to identify things through scenting blood, explains the purpose of their captivity. The fiends' blood can neutralize the Plagues, potentially preventing them from spreading and saving communities throughout Brekon. The very people blamed for the world's suffering may be the only ones capable of stopping it. The discovery shocks the fiends, who have spent their lives being treated as dangerous and unwanted. Esti volunteers to help, although she has difficulty participating in the experiments because the sight of the rats brings back memories of her punishment. Varrik also reveals another reason he has gathered fae: Core fae need Latches, individuals whose life force can become permanently linked to theirs. Without such a bond, their powerful magic gradually destroys them. Esti learns that Varrik's magic has L latched onto her life force, creating a permanent connection between them. Their attraction is tied to this bond, and Varrik insists that she cannot simply walk away from him. Esti rejects the idea that magic should determine her feelings or future and remains focused on returning to her family. For the next couple of days, things are tense

Despite Varrik's warnings, Esti attempts to escape by disguising herself as a servant and leaving the castle. She reaches a dangerous area where a monstrous creature attacks her. Varrik arrives in time to kill it, but another creature wounds him with venom, causing him to collapse. Esti realizes that she cares about him far more than she wants to admit. She stays with him, tends to his wound, and helps him survive the night. As she fears losing him, she finally acknowledges her feelings. When Varrik recovers, they share a kiss, and he admits that his behavior toward her has been cruel. He promises to make an effort to treat her differently. They return to Penbroge, where their relationship begins to change. Varrik gives Esti more freedom, arranges a romantic dinner so they can get to know one another, and allows her to have a private room of her own. A trusted healer also treats her back, finally removing the pain and lasting damage from her whipping. Esti begins to see that Varrik's actions are not entirely motivated by possession. He is trying to protect the fiends and find a way to save Brekon, even though he has made decisions that have hurt the people he claims to be helping.

As Esti spends more time with Varrik, she learns about his broader efforts to establish a place where fiends can live without persecution. She visits Wystinn Burg, a sanctuary where fiends and fae can coexist, and sees that the people Varrik rescued are building lives for themselves. She also learns more about the Core fae who need Latches to survive. Esti confronts Varrik when she discovers that Theo has been imprisoned after trying to escape. She argues that he should not be held indefinitely, and Varrik eventually agrees to release him. Esti is able to say goodbye to Theo and move forward from their past relationship. Meanwhile, Varrik continues responding to Plague outbreaks. When an Ash Plague threatens Beddig Burg, he uses his magic to rescue hundreds of people buried or trapped by the ash. The effort exhausts him, and Esti helps him recover. Watching him risk himself to save strangers strengthens her trust in him. Their feelings become mutual and explicit, and they sleep together. Esti accepts their Latch and begins to believe that she and Varrik can build a relationship based on trust rather than control.

After Esti receives a letter from Aunt Lenna explaining that Marryn's condition is worsening, Varrik agrees to take her back to Hamelyn. He brings a group that includes the healer Lord Conrad, hoping that Conrad can treat Marryn. Esti is relieved to return home and reunite with her family, but their arrival is immediately complicated by Lord Havlon and his patrolmen, who object to Varrik's presence and accuse Esti's family of cooperating with a rival power, thus committing treason. During the confrontation, Marryn's condition changes dramatically. When Varrik approaches her, a surge of magic erupts, and pale antlers begin growing from her head. Varrik realizes that Marryn is a Bane, one of the supposedly defeated beings responsible for the Plagues. The revelation changes everything: Esti's mother is not simply a sick woman who has survived the Break, but a member of the very group the Core fae have spent decades fighting. Varrik reacts with anger and a sense of betrayal, while Esti is stunned by what is happening to her mother. The situation descends into chaos as Havlon's men move to arrest Marryn and Lenna. Esti tries to stop what is happening, but Varrik uses his pipe to compel the patrolmen and take control of the situation. He forces Esti to leave her family and return with him. Esti is devastated that the man she has come to trust is separating her from her mother and aunt, and she is left questioning whether his earlier promises were ever compatible with the truth about his plans. The words she read in his stars when they first met “Defender. Thief. Liar” now seem to describe exactly what has happened.