"Hey, you said you wanted an little sister, right?" First year high school student, Toujo Basara, was suddenly asked by his farther and fell into a state of panic. On top of that, the eccentric father told him he will be getting remarried. He then departed overseas after bringing Basara two beautiful step-sisters. But Mio and Maria's true forms are actually the newbie Demon Lord and a succubus!? Basara was almost entered into a master and servant contract with Mio, but a "reversed" contract was formed by mistake, and Basara is now the master!? Moreover, Basara is being hit by ecchi situation one after another due to the contract, but Mio's life is being pursued by other demon tribes and hero tribes!! The most powerful contractor's desire action drama begins!

Shinmai Maou no Testament was published in English as The Testament of Sister New Devil by Seven Seas Entertainment from January 19, 2016 to July 31, 2018. It has also been published in Italian and Brazilian Portuguese by Panini Comics under the Planet Manga imprint since June 6, 2015 and July 2016, respectively.