The Prince Is A Girl: The Beast King's Captive Mate

Emeriel has disguised herself as a prince her whole life to survive in a dark age where human kingdoms offer women to the Urekai. When her sister is sold to the Urekai, she follows as a slave. After arriving at the Urekai fortress, she experiences a strange burning sensation and is told she is a Siren — a human woman capable of mating with a beast. Her sister survives a night with the feral High King Beast, but when Emeriel secretly visits her and embraces her, the beast catches Emeriel's scent. He breaks free from his chamber, finds Emeriel in the middle of another heat, and claims her as his captive mate. As he closes in, she is told that her only chance is to submit and hope he does not drink her blood.