Fog

The original combat-prevention spell, and the template every Fog effect since has worked from. For one mana, this does something quietly radical: it does not target, it does not care about the attacker's source or color, and it answers an entire combat phase after the attacker has already committed everything. The attacker has tapped creatures and the defender has declared blockers or declined to; the green player spends a single mana and erases that whole phase's reason for existing. That is an enormous amount of structural work for , and the reason no printing has dared price the effect lower. The spell resolves on the stack like any other instant, so it is not unanswerable: Counterspell stops it, and cards that state damage cannot be prevented (Skullcrack, Questing Beast) walk right through it. But absent those, a turn of attacking simply evaporates. The shell has been iterated relentlessly since: Holy Day moved the effect to white, Moment's Peace stapled on flashback to buy two turns instead of one, Tangle added a tap-down rider, Settle the Wreckage rebuilt the idea as a punishing white sweeper. The card itself is plain to the point of austerity, but the design space it opened (combat as a phase you can refuse to participate in) is one of the deepest veins green has ever been handed.