Mercadia's Downfall — ruling 1

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

Mercadia's Downfall : règles, fonctionnement et interactions (MTG)

Mercadia's Downfall is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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

How it works : Mercadia's Downfall

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
The creatures that are attacking is determined when this resolves and is not changed later, even if you have another combat phase.
No other card is named in this ruling.