Whenever Iron Mastiff attacks, roll a d20 for each player being attacked and ignore all but the highest roll.
1—9 | Iron Mastiff deals damage equal to its power to you.
10—19 | Iron Mastiff deals damage equal to its power to defending player.
20 | Iron Mastiff deals damage equal to its power to each opponent.
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.