"Shinmai Maou no Testament," also known as "The Testament of Sister New Devil," is a Japanese light novel series written by Tetsuto Uesu. It follows the story of Basara Toujou, a high school student who unexpectedly becomes the guardian of two demon girls, Mio Naruse and Maria Naruse. The series blends elements of fantasy, action, and romance as Basara navigates his new responsibilities while dealing with various supernatural threats. The narrative explores themes of family, loyalty, and the struggle between good and evil. As Basara protects Mio, who is the heir to a demon lord, he must confront rival factions and uncover secrets about his own past. The series has been adapted into manga and anime, gaining popularity for its engaging characters and dynamic storytelling.