As much as I like this idea... I think it is stupidly powerful.
The only way to do this kind of damage reduction to your opponents creatures is with a Diamond Shield... for comparison:
Diamond Shield
Cost: 7
Ability: Incoming physical attacks are reduced by 3 damage.
Countered by: Steal, momentum, explosion, blessing, chaos power, rage potion, pulverize...
Maximum in play: 1
Black Cat
Cost: 5
Ability: Causes all enemy creatures to do 1 less damage.
Countered by: Rewind Time, Lobotomize, Mutation / Sniper & Otyugh (maybe)
Maximum in play: 6... (with fractal - a lot)
Sorry, sg. Need's a mega nerf in my humble opinion!
Maybe make it a permanent. That stays on your side of the board. Gives -1 to all enemy creatures. This way, there is no fractal abuse, and no 'play this card, but put it over there' stuff... Also, if your opponent uses Steals/Explosions, he would have to make a choice between stealing the Bad Luck, Shield, Weapon or another Permanent (of which there are many choices - empathic bonds/eclipses/shards etc). Think of it as a negative eclipse affecting all enemy creatures