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It's entirely possible you'll only ever see one Mythril Mirror, but it's a doozy of an Item. Wall is a Spell of questionable use since you have to level it quite a bit before it does anything, but an Item that casts Wall 16 is always good. This will make the user (not the full party) invulnerable to black magic of any level, basically shutting down an opponent's casting. You can only use it once, though, so use it wisely. (Note that the Wall bug doesn't affect players the same way it affects Monsters, so you won't be any more vulnerable to death Spells as a result of using this Item.)