There are two places in there game where a very strange man hanging from the ceiling by his feet, stares at you and ignores you. Later in the game, after you use the Evil Plate and open the door, he will talk to you. The man will now take you (whenever desired), to the Dark Realm. In the Dark Realm, killing monsters until two portals open; one to go up and one to go down. Keep going down to the last level (20th) while collecting all the items in route. There will be no monsters to kill on the last level. Open the box to get the Bishamon O flute. Keep the flute just before the last fight with the Demon King.