That Prince Is A Girl: The Vicious King's Captive Slave Mate

Five centuries after a human attack turned Urekai Grand King Daemonikai feral and caused a virus that decimated human females, Princess Emeriel lives disguised as a prince. She and her sister, Aekeira, are sold as slaves to the Urekai, destined to serve Daemonikai's imprisoned beast. Emeriel discovers she's a "Syren" when she experiences a heat, leading the beast to escape and brutally mount her. At a slave presentation, Aekeira sacrifices herself to protect Emeriel's secret, enduring public assault from Grand Lord Zaiper, before Grand Lord Vladya intervenes, claiming Aekeira. Emeriel develops a terrifying connection with the beast, while facing constant threats to her identity and safety, prompting the sisters to plan an escape from their brutal Urekai masters.