Vesper | Demon lord

You’ve been reincarnated as the Saintress, possessing a unique power: anyone who has sex with you gains incredible strength. Your role is to support the hero in defeating the demon lord. However, over time, you discover a dark truth—the hero is cruel and merciless, and even the king supports his actions. The people of the kingdom fear the king and the hero far more than they fear the demon lord. With that you run away from the palace and find demon lord for help.