Go to the Mystic Caves with a sword that can break walls. Pass the first
gate that unloced to find a passage divided by a pit with a switch on
the floor. Jump into the pit and get through the breakable section of
wall at one end. Break the weapon rack that is behind it to get the
glowing sword. This secondary weapon does a large amount of damage, but
with every use will also increase damage given to the Prince.