Hime to Onna Yuusha ga Musubareru tame no 12 no Seikoui

When Princess Sephilia and the fearless heroine Lenaria were young, they pledged their futures to one another. But when Lenaria returns triumphant after slaying the demon lord, it’s not the princess’s hand in marriage she requests—it’s her royal panties! Cursed by the demon lord in its final moments, Lenaria must now perform twelve wildly perverted acts to lift the spell that threatens her very soul.

(Source: Seven Seas Entertainment)

Note: Chapter count includes an omake chapter.