Mass Diminish — ruling 1

If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.

Mass Diminish — ruling 2

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

Mass Diminish — ruling 3

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

Mass Diminish — ruling 4

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Mass Diminish — ruling 5

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

Mass Diminish — ruling 6

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

Mass Diminish — ruling 7

Mass Diminish overwrites all previous effects that set a creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after that ability resolves will overwrite this effect.

Mass Diminish — ruling 8

Effects that raise or lower a creature's power and/or toughness, such as the effect of Titanic Growth, will apply to that creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness.

Mass Diminish : règles, fonctionnement et interactions (MTG)

Mass Diminish 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

Until your next turn, creatures target player controls have base power and toughness 1/1. Flashback (You may cast this card from your graveyard for its flashback cost. Then exile it.)

How it works : Mass Diminish

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."
No other card is named in this ruling.
#3March 19, 2021· Wizards of the Coast
You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.
No other card is named in this ruling.
#4March 19, 2021· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
No other card is named in this ruling.
#5March 19, 2021· Wizards of the Coast
A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.
No other card is named in this ruling.
#6March 19, 2021· Wizards of the Coast
You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.
No other card is named in this ruling.
#7August 23, 2019· Wizards of the Coast
Mass Diminish overwrites all previous effects that set a creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after that ability resolves will overwrite this effect.
No other card is named in this ruling.
#8August 23, 2019· Wizards of the Coast
Effects that raise or lower a creature's power and/or toughness, such as the effect of Titanic Growth, will apply to that creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness.
No other card is named in this ruling.
#9August 23, 2019· Wizards of the Coast
Mass Diminish targets the player, not their creatures. A creature with hexproof or protection from blue may be shrunk this way.
No other card is named in this ruling.
#10August 23, 2019· Wizards of the Coast
Mass Diminish affects only creatures the target player controls at the time it resolves. Creatures they begin to control later before your next turn won't be 1/1.
No other card is named in this ruling.
#11August 23, 2019· Wizards of the Coast
Because damage remains marked on a creature until it's removed as the turn ends, nonlethal damage dealt to a creature the target player controls may become lethal once its base toughness becomes 1.
No other card is named in this ruling.
#12August 23, 2019· Wizards of the Coast
In a multiplayer game, if you leave the game after Mass Diminish resolves but before your next turn begins, its effect lasts until your next turn would have begun. It neither expires immediately nor lasts indefinitely.
No other card is named in this ruling.