itch.io

It looks like it doubles its damage unless you cover all three highlighted spaces. And it has to be all three—just covering one or two gives the same effect as not using any.

EDIT: Also it's an absolute pain in the butt if it's accompanied by enemies that heal it or grant it block, especially if there's multiple such enemies, especially if you have two such encounters right before the final boss, leaving you with basically no HP. (You can probably guess how my latest run ended.)