18+ Put Your Sword Away in the Bedroom

Assassin Yurian is given a deadly mission: marry Duke Larkan of the enemy nation, and then kill him. But Larkan is no ordinary target. Each night, he becomes a wild, lust-driven immortal beast! When Yurian's assassination attempt fails, Larkan offers her a deal: He'll help find her missing brother—if she can succeed in taming him. "If you want to kill me... tame me first." In the dark of night, an intense and dangerous connection is forged. Yurian fights to kill him; Larkan longs for death. Yet the more they clash, the deeper their forbidden instincts pull them toward one another...

(Source: Naver, translated)