I don't know how many folks out the remember Ultima V, of for those that do, the character Grendel. He was this alchemist that tried to concoct a spell to turn a whole band of enemies into rats. One explosion later, things didn't turn out so well for him. He wound up a giant rat that lived in a hut in a swamp. Since then he did manage to come up with a spell to turn one being into a giant rat.
It only worked in combat.
And it only worked on the caster, turning that person into a rat.
Most. Fun. Spell. Ever.
While it didn't enhance your attack or defense, I seem to recall that it did help keep you from getting possessed by certain enemies, which came in handy. Plus you turned into a giant rat for a while. What was not to love? I spammed the freakin bejeebers out of that spell in the game.
So this is kind of what I imagined would happen at one point in the game.