The Testament of Sister New Devil

After ending the battle against Zolgia, Basara regains his peaceful life with his two little sisters, Mio, the daughter of the previous demon lord, and Maria, her little sister who's also a succubus. As Basara pursues the preparations for the sports festival at school, one day he's attacked by a person being manipulated by magic. Recalling the strange gaze he felt at school, a suspicion of his begins to grow...

The day of the sports festival is finally here. Though Basara remains cautious of the mysterious enemy who manipulates people with magic, he happily enjoys the sports festival with Mio and Yuki as the three of them compete in the four-legged-race and enjoy their youth getting entangled and sweaty with each other. Meanwhile, Kurumi, who was placed in charge of guarding the school, discovers a suspicious individual. At almost the same time, a giant twister forms on the sports field. Basara fights to protect the students... but an unexpected individual appears!

After taking Kurumi hostage, Ornis robs Basara of the right arm wielding Bynhildr. Though he lost conciousness from the pain, his sealed power unleashes and goes on a rampage. Their positions are reversed as he corners Ornis with his overwhelming power. However, the moment he tries to silence Ornis for good, a golden chain appears and binds him. When Ornis turns around, he finds Hasegawa Chisato. What secrets has she kept hidden?

As the clash between the peaceful faction and the present demon lord faction draws near, Basara visits the peaceful faction along with Mio. Though Basara enjoys their reunion with Zest and Shella, Maria seems to be acting odd. In fact, a heavy punishment was awaiting her for her betrayal of the peaceful faction. Basara begs Rukia for her forgiveness, and ends up administering the punishment in her stead... but that punishment was to spank Maria's behind while she was in a state of arousal?!

Ramsus is out to extract the "Demon Lord's Power" that Mio possesses, and Basara reveals his anger at Ramsus' lack of concern for Mio's safety. Just as sparks are about to fly, Shella interrupts them. She leads Basara into the bath to try and help calm him down, and when Zest appears, she orders Zest to massage and wash Basara's body with her bountiful bosom. Then Shella tries to get Basara and Zest to forge a master-servant pact...

Basara and the others recieve a message from Lars, calling them to Leohart's royal castle. Then Basara's father, Toujou Jin, appears before them all. He brought with him another message from Leohart. A message saying that they　would hold a tournament before Leohart and Ramsus' forces to determine authority of the demon realm--authority of the demon realm being synonymous with Mio's power. After being selected as the representatives, Basara begins　learning "how to win" from Jin.

Basara and the others finally face the current demon lord, Leohart. Leohart's confidence in his strength fills the air as he welcomes them in without any tricks. Can they really beat him? After Maria encourages them to do what they can for now, Basara commands Mio, Yuki, Maria, Kurumi, and Zest to strip for him. Using items full of aphrodisiacs, they lick, molest, and entwine together...

The group battle between Leohart and Ramsus's forces finally begins. After Mio wins the first match, Maria enters the arena for the next round. Her opponent is Lars. Maria uses her enormous power to face him in close combat, but she fails to overcome the gap in experience and magic power, and gets driven into a corner by Lars' clever attackes. What does Maria pull out in hopes of overturning the battle...? Meanwhile, the demons lay a cowardly trap for Kurumi.

The only remaining match of the tournament is the duel between leaders, Basara vs Leohart. Basara finally appears after his long time in hiding, and faces off against Leohart. The two demon swords clash in fierce battle. Though Basara manages to keep up with Leohart, Leohart clearly holds an overwhelming advantage. Basara is unable to use his Banishing Shift due to the large opening it creates. What secret strategy does Basara pull out in the fight...?

The council's rampage sends quakes through the demon realm, and the demon Chaos is called forth through a giant magic circle. That commander of all heroic spirits flails madly, seeking to claim Basara, Mio, and Leohart's lives. Do they even stand a chance of victory with this supreme being before them?! Meanwhile, Maria, Yuki, and the others put a final end to their own personal battle. "Believe in me... once more." The ultimate sibling combo risks their lives to face the disaster striking all of the demon realm!