The Monster Duchess and Contract Princess (Pre-serialization)

This is a promo for the original novel.

Leslie’s life as the youngest daughter of the Sperado noble family was always used for the betterment of her sister, Eli, who is betrothed to the Crown Prince of the kingdom. After a failed attempt to extract the black magic within Leslie by sacrificing her to transfer it to Eli, Leslie knocks on the door of the monster Duchess Salvatore and, battered, bruised and desperate, pleads for the Duchess to adopt her?!

Original Web Manhwa