The Muramasa blade is probably the easiest to find, of the best weapons. It is in Rapax Castle, you just have to find the right deposit slip, to use in the kings quarters, to get it.The Light Sword is probably the most difficult to get. There are two places to get it in the Bayjin area: one of the huts treasure chests and the encounter (in Bayjin shallows) with the ghosts near the sunken ship. There is probably only about a 1% chance to get it at either spot.
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Tools for Wizardry(r) 7