Basara’s step-sister Naruse Mio possesses the power of a demon lord--a power that is growing stronger by the day, luring demons to Earth. Basara has forged a veritable harem at the Toujou household, living an ideal life together with Mio, the succubus Maria, his two heroic childhood friends, and their demonic servant Zest. But now all that is under threat. What will become of Basara and Mio’s relationship if she is forced to live in the underworld itself? The Testament of Sister New Devil: STORM concludes in this exciting volume!