Force of Despair

The Force cycle's black entry, and the one whose free-cast condition asks the most of the effect wrapped around it. Every member of the cycle lets you pitch a same-colored card to cast it for free on the opponent's turn, which turns a three-mana instant into a zero-mana ambush; the design bet is that the effect has to be narrow enough that free access does not break the game. Here the constraint is temporal rather than restricted by target: it kills only creatures that entered this turn, which makes it a punish card, not a sweeper. It answers the turn where an opponent floods the board (a token swarm, a mass reanimation, a cascade of hasty threats resolving into the same combat step) and does nothing against a battlefield that was already sitting there. That specificity is what pays for the free cast: an unconditional wrath you could deploy for a discarded black card during the opponent's declare-attackers step would be absurd, but a sweeper that only sees this turn's arrivals is a reactive tool that has to be aimed. The reward for aiming it correctly is a tempo swing no other Force offers: the opponent commits their turn to building a board, you exile a card to erase all of it before they can attack, and the mana you never spent is still available for your own turn.