Tidal Flats

{U}{U}: For each attacking creature without flying, its controller may pay {1}. If that player doesn't, creatures you control blocking that creature gain first strike until end of turn.

Fallen Empires

o
Uoo
U All your creatures that are blocking any non-flying creatures gain first strike until end of turn. The attacking player may pay o1 for each attacking creature to prevent Tidal Flats from giving that creature's blockers first strike.

Can be used more than once in a turn. Each time, the opponent can pay to prevent giving first strike to a creature, but this payment only stops the current activation and not future activations of Tidal Flats.

First Strike is granted to your creatures currently blocking non-Flyers when this effect resolves.

The payment is made when the Tidal Flats effect resolves.