Knightfall Trap

If three or more creatures are attacking you, you may pay {w} rather than pay Knightfall Trap's mana cost.
Untap all creatures you control. Creatures you control get +2/+1 and gain first strike until end
of turn. Knights you control get an additional +1/+1 until end of turn.
The sandstorm of dust, the clattering of hooves, the unsheathing of steel. The cavalry has arrived.