Well, if it's anything like any other shield (see fire shield, for example), it would only affect critters attacking the player who's behind the shield, and activates just after they hit.
So an Armagio attacking you when you have this shield would do 1 damage and turn into a 25/1 that's easy to take care of but will hurt like hell next turn if you don't.
Pretty awesome indeed. Perfect match for Maxwell's Demon.