Arcum's Whistle
Oracle Text
, : Choose target non-Wall creature the active player has controlled continuously since the beginning of the turn. That player may pay , where X is that creature's converted mana cost. If they don't pay, the creature attacks this turn if able, and at the beginning of the next end step, destroy it if it didn't attack this turn. Activate this ability only before attackers are declared.
Card Rulings
9/20/2013 If a turn has multiple combat phases, the ability can only be activated before the beginning of the declare attackers step of the first combat phase in that turn.
If a creature becomes tapped or does not untap before its next combat, then it will be destroyed if its casting cost is not paid.
Thalakos Dreamsower, Island Sanctuary, Giant Oyster, Halls of Mist, can give the whistle a death toll.
But it is not going to be effective late games, or against low casting cost and token creatures where the effect is negated with out much cost.
But dont underestimate the trickery that can be played on someones mana greed, if you leave an opening for attack they may not even pay a low casting cost. Then surprise them with an instant effect a cantrip that otherwise would not be as effective.