Slutty Princess Diaries(2004)

Slutty Princess Diaries is an adult anime series based on the erotic game by Nel. After his mother's mysterious death, swordsman Distol vows to become the world's strongest—only to face fresh tragedy when his sister, Princess Qoona, is chosen as the Holy Maiden and must sacrifice herself to seal away the demon king. Desperate to prevent losing another loved one, Distol accepts a dangerous pact with a mysterious witch. In exchange for immense power, he must infiltrate the nation of Bastarauge, steal its treasure, kidnap Princess Elena, and corrupt her from within. What begins as a quest to save his sister spirals into a dark conspiracy that threatens kingdoms and tests the limits of his ambition and morality.