Fight through all 20 levels of the Dark Realm. Kill all of the monsters on the 20th level, then open the treasure box to discover the Bishamon Ocarina. In the area just beyond the second Marcellus boss fight, use the ocarina on the bone door to open it. Head inside and claim the prize: A sword with unlimited magic which kills any non-boss character in a single swipe. A very powerful ally indeed.
Please do the following:
In the Dark Realm(where you fight through levels of demons), one of the 'prizes' for the 'level' is a talisman, supposedly the 2nd out of 2 in the game. However, if you quit and leave right after getting it, and then go back, beat that 'level' again, and it should be there again. Repeat as necessary.