Demon's Sword Master of Excalibur School

According to this tale, the demon king Leonis imprisoned himself for a thousand years to get ready for the decisive fight, but when he emerged, he discovered that he was now inhabiting the body of a 10-year-old kid. Then, a stunning student from the Holy Sword Academy by the name of Riselia asks him, "Why were you imprisoned in such a place? Now everything is OK; your big sister will take care of you." Leonis, who is under Riselia's custody, is dismayed to discover how drastically things have altered. Leonis finds himself enrolling in the Holy Sword Academy, bewildered and puzzled. What gonna Leonis do?