The Cursed Kingdom by Invai Wright

Here is a summary with spoilers for The Cursed Kingdom by Invai Wright

Abby Williams has been best friends with Lillian Collins since childhood. When Lillian first arrives in their small town, Abby is initially jealous of her because Abby feels that Lillian is taking attention away from her brother. Lillian is noticeably different from the other children, with white hair and violet eyes, but the two girls eventually become friends. Lillian eventually tells Abby the truth: she is a faerie who came to the human realm with her mother, Callie. Lillian and Callie had been hiding from the faerie world for years. Lillian's mother eventually dies, leaving Lillian alone in the human realm. Despite knowing that Lillian is a faerie, Abby remains her closest friend and continues to protect her secret.

As adults, Abby and Lillian live together. Lillian's body is deteriorating because the human realm contains almost no magic, which faeries require to survive. The only thing keeping her alive is delysum. In the faerie realm, delysum is generally known as an extremely dangerous substance because it amplifies magic to the point that it can overwhelm and kill a faerie. In the human realm, however, there is almost no ambient magic for it to react with. Because Lillian has only a small amount of magic remaining inside her, consuming small amounts of delysum allows her to survive rather than killing her. Her supply is running out, though, and Abby watches Lillian become increasingly weak. Lillian refuses to return to the faerie realm, knowing that going back would expose her identity and put both of them in danger.

After Lillian's condition becomes particularly bad, Abby decides that she cannot simply watch her friend die. Lillian refuses to help Abby go to the faerie realm, so Abby secretly makes her own plan. On Abby's birthday, she gets Lillian to drink alcohol and waits until Lillian is drunk enough that she agrees to open a portal. Abby has already packed supplies and prepared for the possibility that she will not return. Once Lillian opens the portal, Abby takes the opportunity to jump through it before Lillian can stop her. The portal closes behind her, leaving Abby trapped in the faerie realm and Lillian in the human world. Abby's entire purpose is to find delysum and bring enough of it back to keep Lillian alive.

Abby initially has no idea how different the faerie realm is from what she has imagined. She spends a considerable amount of time traveling through unfamiliar countryside, dealing with hunger, exhaustion, weather, and the danger of being alone in a world she knows almost nothing about. Eventually she encounters Samuel, a faerie who is willing to help her. Through him, Abby begins learning the customs of the faerie realm. One important custom concerns physical contact and mate bonds. Faeries can discover a fated mate through touch, and gloves are commonly worn to prevent accidental bonding. Abby does not understand these customs when she first arrives, creating several awkward situations. Samuel gives her food and information and helps her understand where she is and what is happening politically. He also discovers that Abby is not his mate when they attempt to determine whether a bond exists.

Samuel explains that the faerie kingdom is in a dangerous political situation because of the shifters. The shifters have been pushed away from the faerie kingdom and are connected to the Redstall Forest, an area that is dangerous and heavily restricted. Delysum grows in the forest, but the plant is not simply a medicine. The faeries believe the shifters are cultivating or gathering delysum because it can be used as a weapon against faeries. The faeries and shifters are therefore close to open war. Abby realizes that the simple plan she had imagined  (enter the faerie realm, find a plant, and leave) is much more complicated than she expected.

Abby eventually makes her way toward the Redstall Forest. The forest is separated from the faerie territory by a guarded wall, and Abby has to sneak through rather than simply walk in. She crosses into the forest and continues searching for delysum. While she is there, she encounters two powerful men: Kieran Ashford, the faerie crown prince, and Mason Verlice, a shifter prince who is living among the faeries. Kieran and Mason have their own complicated history. They are connected by a forked mate bond, meaning that the two men share a bond that is supposed to connect them to the same eventual mate. Their relationship is strained by their political positions and by the conflict between faeries and shifters.

Kieran and Mason capture Abby. They quickly realize that she is a human who has somehow entered the faerie realm and made it into the Redstall Forest. Abby refuses to tell them the real reason she crossed into the realm because she is still trying to protect Lillian. She also does not want them to discover that she has a connection to a faerie living in the human world. The princes are suspicious of her, but they also realize that a human could potentially be useful to them. Abby is weak compared with the faeries and shifters around her, but she becomes a bargaining piece because Kieran and Mason have plans involving the goddess Zaha.

The journey through the forest becomes increasingly dangerous. Abby is forced to travel with Kieran and Mason rather than being allowed to continue searching for delysum on her own. The three of them encounter the dangers of the forest, and Abby is repeatedly placed in situations where her survival depends on the two princes. Although Kieran and Mason are technically her captors, their treatment of her is not entirely consistent: they threaten her and keep her prisoner, but they also protect her from the dangers around them. Abby's relationship with both men becomes increasingly complicated as they spend more time together. Kieran is more controlled and politically minded, while Mason is more openly hostile and physically intimidating. Neither man initially knows that Abby's journey is actually connected to Lillian.

Eventually Kieran and Mason bring Abby to the portal leading to the gods' realm. Their intention is to use Abby as an offering to Zaha and obtain her help in dealing with the threat posed by the shifters and the coming war. Zaha, however, does not want Abby. The proposed trade fails. Instead, Zaha offers the princes a different bargain. Kieran must surrender his claim to the crown and allow Mason to take his place in exchange for Zaha's intervention. Kieran accepts the bargain. This means that the political balance between the faeries and shifters changes dramatically, with Mason positioned to become the ruler instead of Kieran. Abby is not kept by Zaha and is sent back toward the human realm, but Kieran accompanies her.

When Kieran returns to the human realm with Abby, he discovers Lillian. Kieran immediately recognizes Lillian as the faerie who disappeared years earlier with her mother. His personal history makes the discovery especially serious: Lillian and Callie were connected to the events surrounding the death of Kieran's father. Kieran takes both Abby and Lillian prisoner. Abby is separated from Lillian and tries to protect her, while Kieran and Mason focus on determining exactly what happened years earlier and why Lillian has been hiding in the human world.

Lillian is interrogated about Callie and about Kieran's father's death. Kieran believes that Lillian and her mother were responsible for his father's murder. His understanding is that Lillian gave Kieran's father delysum-infused tea, which caused the delysum to overwhelm his magic and resulted in his death. The effects were severe and rapid, and Kieran has believed for years that Lillian deliberately poisoned his father. Mason also has personal history with Lillian. He remembers her from childhood and was once close to her, which makes the situation more complicated than a simple interrogation of an enemy.

Lillian eventually explains that Callie is dead. She says that Callie died because they had spent so long in the human realm without sufficient magic. Lillian herself survived because she had a supply of dried delysum leaves from the time she and her mother left the faerie realm. There was never enough for both of them, so Callie used very little and eventually died, leaving the remaining supply for Lillian. Lillian has been using the delysum in tiny amounts because there is almost no magic in the human realm for it to amplify. This explains why the substance that the faeries consider deadly has actually been keeping Lillian alive for years.

Lillian also reveals that Abby came to the faerie realm because she was running out of delysum. Abby did not know the full history of the plant or the danger surrounding it. She simply knew that Lillian needed it to survive. This means Abby's entire journey into the faerie realm was based on an incomplete understanding of what she was looking for. Kieran and Mason realize that Abby nearly died because she entered the Redstall Forest without knowing how dangerous it was. Kieran also has to reconsider what he believes about Lillian's role in his father's death, because Lillian's explanation of delysum gives a different context to the substance.

The revelations about Lillian's mother also expose the larger political history behind the conflict. Callie had been involved with the faerie and shifter political situation before she and Lillian disappeared. Kieran's father's death, the shifters' use of delysum, and Callie's actions are all connected to the growing hostility between the two kingdoms. The princes therefore have to deal with two separate problems at once: the personal history surrounding Lillian and Callie, and the much larger conflict that is pushing the faeries and shifters toward war.

After Lillian's explanations, the situation changes again when the mate bond activates. Lillian is revealed to be the mate connected to Kieran and Mason. This is significant because Kieran and Mason had already been connected by their forked mate bond, and Lillian's appearance means the bond they had been expecting has finally manifested. The revelation changes how the two princes view Lillian and creates an immediate complication for Abby, who has spent the entire story risking herself to save Lillian and has also developed complicated feelings toward Kieran and Mason.

The final portion of the book brings the personal conflict into the faerie court. Queen Gitta, Kieran's mother, becomes involved in determining what should happen to Lillian, Abby, Kieran, and Mason. Kieran has already sacrificed his claim to the crown because of his bargain with Zaha, and Mason's position has therefore changed. At the same time, Lillian is now connected to both princes through the mate bond, despite the unresolved questions surrounding her mother's actions and Kieran's father's death. Queen Gitta's judgment leaves the characters' positions unresolved while the conflict between the faeries and shifters continues to escalate. The book ends with Abby, Lillian, Kieran, and Mason caught in the consequences of the revelations about the mate bond, the delysum, Kieran's lost crown, and the approaching conflict between the two kingdoms.