When i use it nothin' happens, is there something you have to do, like Blitz?

Celes's Runic command allows her to absorb the next spell cast, turning it into MP for her to use. This ability works on the next spell that is cast, so it could absorb a friend or foe's spell. Furthermore, Celes must have a Runic capable sword equipped for this to work; the menu item for Runic is grayed out if anything else is equipped.

Interestingly, Runic reverses normal elemental attributes. If Celes absorbs a spell she would normally take only half damage for, she gains half the MP, while Immunity to an Element gives her no MP and normally absorbing an Element makes her actually lose the MP for the spell.