I know "under" isn't correct, but I need to think of a good verb for it.

Once the creature attacks or blocks, do you flip this face-up and cast it once, so that it's no longer associated with the creature? Or is it instead an Illusionary Mask spell? "I destroy your creature." "How?" "With this spell that I don't need to show you."

The intent is that you reveal the spell, casting it from exile, and when it resolves, it goes to your graveyard.

That's what I assumed. It should probably say "Whenever that creature attacks or blocks, you may reveal this card and cast it without paying its mana cost" just to be on the safe side.

I agree both that the current wording works ("cast" implies "put it onto the stack", which implies face-up unless specified otherwise) and that it'd be worth spelling it out explicitly anyway.

I assume all Subterfuge spells will have Subterfuge ? (As otherwise you could tell them apart by the cost paid.) Might it be worth moving the inside the keyword, like in morph?

Also, I note that a green-white or blue-red deck could happily subterfuge this card. That doesn't seem right.

"As long as you control a Swamp?" Hmm.

Maybe this mechanic should take a cue from Morph, and have a de-Subterfuge cost of .

Then again, looking at the cost and the subterfuge cost, it looked like having green-white decks subterfuge this was the whole point. I got to admit, it bugs me too.

Oh, one more thing, modern power levels might suggest this card is an uncommon. Removal on par with Dark Banishing isn't supposed to be common nowadays...

Depends what you mean by "on par with". Common can get widely-applicable removal (Trostani's Judgment, Launch Party), or cheap removal (Smite, Mugging), just not both on the same card.

Yeah, it needs something. "Destroy target creature", in artifact, for ? Crazy!

Ok, nonblack creature; but still.

I need to either limit Subterfuge so that the spells can only be placed on creatures of the proper color, or make the lessening of the color commitment less complete.