MArcomage

Maybe I'm missing something, but I feel like this is kind of weak. Even as a common, I don't think I'd use it. I can think of two main reason for wanting one of your opponent's cards to have a new flag: 1) To activate some kind of trap card (trigger trap comes to mind) - this doesn't apply in this case, since the card isn't quick. 2) To impede your opponent from using a card that has a weaker effect (or none at all) if it has a new flag, like Prince of Thieves or Restoration. This just prolongs the pain by one turn, so it's not all that great...

I was thinking the same. The only situation I can think of where this would be helpful is if that extra turn without Prince being played secures a resource accumulation victory for yourself, and that's such a specific set of conditions that I doubt anyone would grant it a deck slot.

You were right I forgot to put the quick keyword.

Uses (besides some concepts cards) against: Ambush,Order of the White Lotus,Prince of thieves,Prosperity, maybe sometimes Power surge (delaying some gems)
played before: Don't be hasty, Trigger trap