In the Demon Realm (just after you destroy the Ultimate Creation) is a save mirror and three doors. Go to the door that resembles an iron gate. It will not open at first. Use the Bishimon O flute on it and enter. Go to the Bishimon Sword jutting out of the Skull Demon wall object and take it. It is more powerful than all three of your weapons put together, and it has speed. The sword also has an unlimited supply of magic.