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Coven — Choose target creature you control and target creature you don't control. If you control three or more creatures with different powers, put a +1/+1 counter on the chosen creature you control. Then the chosen creatures fight each other. (They each deal damage equal to their power to the other.)
"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."
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.
If you control three or more creatures with different powers and the target creature you control is still a legal target as Duel for Dominance resolves but the target creature you don't control isn't, you will still put a +1/+1 counter on the creature you control. It isn't much of a duel, but your creature is still dominant.
Only the +1/+1 counter is contingent on having three or more creatures with different powers. The target creatures will still fight even if you don't have three or more creatures with different powers when Duel for Dominance resolves.
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.
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.
