An interesting card indeed... if you managed to get 'swarm' on one of your creatures that you play multiples of (Say, Firefly, Skeleton, or some
creature), then it'd be hilarious.
Opponent: "All I have to do is play Rain of Fire next turn, and his creatures will be screwed over."
You: *plays this card - majority of your creatures gain Swarm*
Opponent: *sees HP boost on a number of your creatures* O_O
Devourer, poisonous, and airbourne/ranged would also be useful in case you're facing someone using Wings or Otyughs, but the question is, can these passive skills stack? (i.e. - since the card has no mention of adding -more- passives to a creature with or without passive skills...)
Oh, and I have idea if your card art is... legal? ??? I've never heard of anyone just using the default background as card art...