Iron Mastiff — ruling 1

Once Iron Mastiff's triggered ability begins to resolve, no player may cast spells or activate abilities until it finishes resolving. Notably, this means that players may not take actions that would change Iron Mastiff's power after they know the result of the rolls but before players are dealt damage.

Iron Mastiff : règles, fonctionnement et interactions (MTG)

Iron Mastiff 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

Whenever this creature attacks, roll a d20 for each player being attacked and ignore all but the highest roll. 1—9 | This creature deals damage equal to its power to you. 10—19 | This creature deals damage equal to its power to defending player. 20 | This creature deals damage equal to its power to each opponent.

How it works : Iron Mastiff

Original English text — Wizards of the Coast / Scryfall

#1June 10, 2022· Wizards of the Coast
Once Iron Mastiff's triggered ability begins to resolve, no player may cast spells or activate abilities until it finishes resolving. Notably, this means that players may not take actions that would change Iron Mastiff's power after they know the result of the rolls but before players are dealt damage.
No other card is named in this ruling.