Afterthought

Enchant target non-land permanent your
opponent controls. During their upkeep,
specifically, they must say a word or phrase you
come up with as a cost. If they don't pay the cost
during their upkeep; you can point it out during
your upkeep, specifically. When you do; phase
out the permanent this card is enchanted to and
deal 3 damage per upkeep that they didn't pay
the cost. If your opponent pays the cost after
having not done so previously; previous unpaid
costs must be ignored.