Purify the Grave

• 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 Coast2011-09-22
  Official Ruling
  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.
• WWizards of the Coast2011-09-22
  Official Ruling
  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.
• 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."
• 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
  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
  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
  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.

Rulings via Scryfall