This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
There are four weapons in the game - the Rune Edge, Ogre Nix, Ragnarok,
and Illumina - which consume MP each time they're used to inflict a
critical hit on the enemy. If you have lots of MP on one character, it
might be worthwhile to try this out.