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.

"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."

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.

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.

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

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

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.

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 targets the player, not their creatures. A creature with hexproof or protection from blue may be shrunk this way.

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.

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.

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.