Diabolo Vol. 1 (Shonen Manga)

Something extra-wicked this way comes! Ren and Rai play with fire when they sell their souls to the demon Diabolo to save Ren's cousin. Diabolo gives Ren the power of ultimate offense and Rai the power of ultimate defense. However, those who fall into the Diabolo's trap start to go crazy at 17, and at age 18 they reach the point of no return...