Quicksilver by Callie Hart

Here is a summary with spoilers of Quicksilver

Saeris Fane lives in Zilvaren, an oppressive desert city ruled by the immortal Queen Madra, where water is tightly rationed and survival depends on deception. Her mother was executed, the city is divided into prison-like wards, and theft is punished with torture and death. Determined to keep herself and her brother Hayden alive, Saeris steals a valuable gold gauntlet from a Guardian. After finding Hayden with Carrion Swift, a flirtatious criminal with whom she shares a complicated history, Saeris entrusts Hayden with the stolen gauntlet. When he disappears, she chases after him and finds Guardians cornering him. To protect her brother, Saeris confesses to the theft, kills two Guardians during the confrontation, and is arrested by Captain Harron.

Saeris is taken before Queen Madra and brought into a chamber containing an ancient sword embedded beside a quicksilver pool. Madra demands to know where Saeris learned to fight, believing she was trained by the mythical Fae. When Saeris refuses to cooperate, Madra orders her execution and threatens to destroy her entire ward. During the execution, Saeris instinctively melts Harron’s dagger using a hidden ability to manipulate metal. She grabs the sword and accidentally activates the quicksilver pool. A mysterious figure she believes is Death emerges, chains her, and drags her through a portal. Saeris awakens fully healed in Yvelia, a Fae realm, where she meets Everlayne De Barra and Renfis Orithian. They reveal the man who brought her through the portal is Kingfisher, also called Fisher, an exiled and feared figure connected to the quicksilver.

In Yvelia, Saeris learns she possesses a rare ability to manipulate quicksilver, a power Belikon, the king of Yvelia and Everlayne’s father, wants to use to reopen pathways between realms. Though Belikon despises Fisher, his wife’s son from a previous marriage, he forces Fisher to help train Saeris. Struggling to control her abilities and desperate to save Hayden, Saeris bargains with Fisher. After distracting him with a kiss and stealing his ring in an attempt to escape, she convinces him to retrieve Hayden from Zilvaren in exchange for a blood oath. Fisher instead returns with Carrion, forcing Saeris, Fisher, Carrion, and Ren to flee Belikon’s court and travel to Fisher’s home in Cahlish. Along the way, Saeris reunites with her fox Onyx and begins learning more about the Fae realm, the war consuming it, and Fisher’s past.

Saeris discovers Yvelia has been fighting a losing war against Sanasroth, a ruined land overrun by vampires. Malcolm, leader of the vampires, created both mindless Feeders and powerful vampire lords. As Saeris trains with quicksilver and creates magical relics, her relationship with Fisher deepens despite their constant conflict. She witnesses Fisher suffering violent episodes caused by the quicksilver slowly killing him and begins understanding the depth of his torment. In Ballard, a peaceful village tied closely to Fisher’s past, she learns about his mother Edina, his responsibilities, and the blood oath that trapped him for more than a century in Malcolm and Belikon’s schemes. Fisher eventually reveals the truth about Gillethrye, where Belikon, Madra, and Malcolm manipulated him into a massacre that led to thousands of deaths. As Saeris and Fisher grow closer, magical runes appear on their skin, revealing they are fated mates capable of communicating telepathically.

Saeris’s power continues growing as she bargains directly with the quicksilver to forge weapons and relics. When the vampire lord Taladaius reveals Malcolm has kidnapped Everlayne and infected her with deadly venom, Fisher agrees to deliver Saeris to Malcolm in exchange for Everlayne’s life. Though Fisher initially rejects their bond because the quicksilver inside him is killing him, he eventually accepts it after finally allowing himself to be vulnerable with Saeris. The next morning, Saeris discovers Fisher has left to sacrifice himself alone. Furious, she follows him to the ruined city of Gillethrye with Carrion and Lorreth. There they discover the full truth about the war. Belikon, Madra, and Malcolm are siblings secretly working together to prolong the conflict while Malcolm builds an undead army from trapped souls and ruined Fae.

Inside Malcolm’s labyrinth, Saeris locates the quicksilver coin while Carrion distracts Malcolm by allowing himself to be bitten, revealing his cursed Fae blood is poisonous to vampires. During the battle, Malcolm mortally wounds Saeris, but she kills him before dying, freeing the trapped souls of the undead Fae. As she dies, Taladaius transforms her into a vampire. Saeris is then summoned by Zareth, the God of Chaos and Change, who warns that her bond with Fisher could trigger the end of the world but also offers hope that disaster can still be prevented. When Saeris awakens, she is no longer human but a unique blend of Fae and vampire. Fisher has finally discovered a way to remove the quicksilver from his body and survive. With Malcolm defeated and the Triumvirate weakened, Saeris is named the future queen of Sanasroth, setting the stage for the next phase of the war and her future beside Fisher.