MM: Mercadia's Downfall, Instant 2R Each attacking creature gets +1/+0 until end of turn for each nonbasic land defending player controls.

Mercadia's Downfall

Each attacking creature gets +1/+0 until end of turn for each nonbasic land defending player controls.

10/4/2004 The creatures that are attacking is determined when this resolves and is not changed later, even if you have another combat phase.