An elite assassin from the Poison Clan is tasked with finding a wife to prevent his sister from being forced into an arranged marriage to produce an heir. He seeks assistance from a notorious marriage swindler in what action-romance anime series?

The anime series you're referring to is "Marriage Toxin," a unique action-romance story that blends the dangerous world of assassins with the comedic challenges of finding love. The protagonist, Hikaru Gero, is an elite assassin from the ancient Poison Clan, one of the Five Great Families of "Masters." Despite his deadly skills and extensive knowledge of poisons, Gero is completely inept when it comes to social interactions and romance, having lived his entire life in the underworld.

The central conflict of the series arises when Gero's clan demands he marry and produce an heir to continue their lineage. If he fails, his younger sister, Akari, who is already in a loving relationship, will be forced into an arranged marriage to fulfill the clan's demands. Desperate to protect his sister's happiness (Review), Gero takes on the seemingly impossible mission of finding a wife for himself.

To navigate the unfamiliar world of dating and relationships, Gero seeks the help of Mei Kinosaki, a notorious and highly skilled marriage swindler. Kinosaki, who is revealed to be a cross-dressing man, is a master of seduction and understands the intricacies of romance far better than Gero. Together, this unlikely duo embarks on a series of adventures, facing not only the challenges of matchmaking but also dangerous encounters with other figures from the criminal underworld. The series, originating as a manga written by Joumyaku and illustrated by Mizuki Yoda, offers an engaging mix of intense action sequences and comedic romantic situations.