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Purify the Grave can't be used in response to a spell being cast with flashback or a spell that requires cards to exiled from a graveyard as an additional cost to try and counter the spell or prevent it from being cast.
If it's your turn, and you cast a spell with flashback from your hand, you'll have priority to cast that spell again using flashback before any other player has priority to cast Purify the Grave in order to remove that card from your graveyard.
"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."
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.
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.
You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.
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.
