Sneak Attack is an action that grants two allies the ability "Sneaky" when you play it. Play this on a few large allies and you can attack without fear of your opponent blocking you! Ally destruction cards and Transmute to Butterfly can still stop you, though-- watch out!. Transmute will even remove Sneaky and allow your opponent to block and kill your ally. Still, this is a valuable finishing card in a board still, similar to Swamp Witch and Slime Rain.This card doesn't effect the battlefield at all on its own, but paired with another card that has a similar effect (like Mist Machine) your opponent will be unable to do anything about your sneaky attackers.
Strong against: Ally-heavy decks
Weak against: Transmute to Butterfly