Just Fate

You can only cast it when a creature is swinging at you, in the one step of one phase where attackers are declared and you have already been attacked. That single restriction is what gives the card its character: it converts a clean white kill spell into a pure punishment for committing to combat. You cannot hold it up as a proactive answer, cannot point it at a tapped threat during your own turn, cannot use it to clear a blocker before damage. The creature has to come at you first. As a teaching tool for new players learning combat, the restriction does real pedagogical work, reinforcing that attacking is a decision with consequences and that the defender holds answers the attacker cannot see coming. The cost of that narrow window is steep: three mana for a conditional, single-target kill that is dead weight outside one specific step is a rate no serious removal spell would accept. But the constraint is also what keeps the card honest, a deliberate exercise in showing how a timing window can price down an effect that would otherwise be too clean. Strip the timing qualifier and this is a plain destroy-target-attacking-creature instant; with it, it becomes a study in how much a single restriction can ask of a card.