Mass Diminish

• WOTC Official Reminder Textofficial rules
  Flashback — Flashback [cost] (You may cast this card from your graveyard for its flashback cost. Then exile it.)

• WWizards of the Coast2019-08-23
  Official Ruling
  Mass Diminish targets the player, not their creatures. A creature with hexproof or protection from blue may be shrunk this way.
• WWizards of the Coast2019-08-23
  Official Ruling
  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.
• WWizards of the Coast2019-08-23
  Official Ruling
  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.
• WWizards of the Coast2019-08-23
  Official Ruling
  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.
• WWizards of the Coast2019-08-23
  Official Ruling
  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.
• WWizards of the Coast2019-08-23
  Official Ruling
  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.
• WWizards of the Coast2021-03-19
  Official Ruling
  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.
• WWizards of the Coast2021-03-19
  Official Ruling
  You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.
• WWizards of the Coast2021-03-19
  Official Ruling
  A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.
• WWizards of the Coast2021-03-19
  Official Ruling
  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.
• WWizards of the Coast2021-03-19
  Official Ruling
  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.
• WWizards of the Coast2021-03-19
  Official Ruling
  "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."

Rulings via Scryfall