I'd considered that, but there'd need to be some kind of ratio to it, otherwise taking a 200+ damage hit and gaining 200+ MP would probably earn itself the balance hammer. As a simple conversion, it does wind up emulating things like the Ethereal Earring, but if it also does nothing to the mob, not much point in calling it a Spikes spell. Damage to the mob's actual MP can be unedited, though. Mobs have a metric fkton of MP, so it's not so much about draining them as it is helping us out a bit when under pressure. I'd also be okay with retaining an element of MP recovery while inflicting a MATK down debuff. Or it could give us a building MATK pool a la Afflatus: Solace and be expended on an action that checks MATK like a nuke or elemental WS.

And while I didn't specify, Renew would only pertain to white and black magic buffs.