Once and Future

If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.

If one of the two target cards is an illegal target by the time Once and Future resolves, you'll put the other in the appropriate zone and exile Once and Future. If both targets are illegal, the spell won't resolve and you won't exile Once and Future.

If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.