The Bishamon O Flute:

Twice in the game a man hangs from the 
ceiling, watching you but ignoring you.
After you use the Evil Plate later in 
the game, the man will talk to you. He
can take you to the Dark Realm, where 
you kill monsters until two portals open
-one leading up and one leading down. 
Keep going down until you reach the 
bottom level. Here you will find a box 
containing the Bishamon O Flute.