The Demon Girl by Penelope Fletcher

Rae Wilder's world is in chaos. With humanity facing extinction as supernatural beings swarm the earth, Rae finds herself pursued by a mysterious fairy who claims they share a demonic kinship. When she ventures beyond the Wall into demon territory, she becomes entangled in a dark realm of magic, fierce creatures, and sacrificial rituals, where she's tasked with guarding a powerful amulet.

As strange changes alter her mind and body, Rae resists her destiny, struggling against the forces pulling her toward darkness. But with an alluring vampire eager for her attention, life grows even more complex. Now, Rae must decide: cling to her humanity or embrace her power, risking transformation into an unstoppable force.