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Until end of turn, you may cast target instant or sorcery card from your graveyard without paying its mana cost. If that spell would be put into your graveyard, exile it instead. Exile Sins of the Past.
The instant or sorcery card is cast from your graveyard, not your hand. You choose modes, pay additional costs, choose targets, etc. for the spell as normal when casting it. Any X in the mana cost will be 0. Alternative costs can’t be paid.
The instant or sorcery card never gets put back into your graveyard, even if it’s countered. You can’t cast it more than once.
