The Testament of Sister New Devil

is a Japanese light novel series written by Tetsuto Uesu and illustrated by Nekosuke Ōkuma. Nine volumes have been published by Kadokawa Shoten since 2012 under their Kadokawa Sneaker Bunko imprint. A manga adaptation has been serialized in Kadokawa Shoten's Shōnen Ace magazine since May 2013. A second manga adaptation has been serialized in Hakusensha's Young Animal Arashi magazine since February 2014. An anime adaptation started airing in January 2015. After it concluded in March, a 13th OVA episode came out in June. A second season premiered in October 2015 with a second OVA to follow in January 2016.

Plot
"Hey, you said you wanted a little sister, right?" High school freshman, Basara Tojo, was suddenly asked this question by his father who tells him that he's getting remarried. He then departed overseas after bringing Basara two beautiful step-sisters; Mio and Maria. But the true forms of the two girl's are actually a newbie Demon Lord and a succubus! Basara was almost forced 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 situations 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!

Official

 * Light novel official website
 * Manga official website
 * Anime official website

Other Sources

 * Wikipedia

Other Wikis

 * Shinmai Maou no Keiyakusha Wiki - Duplicate.