Demon Ball is a move which can be learnt by Rapp . In this move, Rapp throws a ball of fire which turns into an explosion (familiar to the "BOOM!" explosion) hitting one enemy.
The move is learnt at Knife lvl 20, Sword lvl 18 and Projectile lvl 23. It can be used with any weapon equipped. It costs 40 SP to use, which is just under half of the stronger version, Neo Demon Ball, which attacks all enemies.