Endure

A total wall, and the rate is the whole argument against it. For five mana at instant speed, your turn becomes immune: every burn spell, every alpha strike, every direct-damage finisher fizzles against you and against the board you've built. That is a genuinely complete answer, the kind of blanket prevention an aggressive deck simply cannot punch through. The cost is the catch, and it is a steep one. Five mana to develop nothing, to do nothing offensively, to do nothing at all if your opponent has no damage to deal that turn, is an enormous tempo concession in a game where the decks most likely to kill you with damage are also racing to do it before turn five. The narrower fog-class effects that cost one or two mana have always paid for their splash by staying cheap enough to leave up alongside real plays; this asks for a full turn's worth of mana on insurance. Where it earns its keep is the spots a cheaper fog cannot reach: covering your permanents and not just your face means a sweeping burn play or a mass-damage effect aimed at your creatures gets neutralized too, and instant speed lets you wait until your opponent commits the spell or attack before you decide whether the cost is worth paying. A defensive tool built for the rare board state where total immunity for one turn outvalues anything else five mana could buy.