Sword (Bishamon):

In the Demon Realm is a save mirror and 3 
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).