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To determine whether it can be countered and its damage can be prevented, Banefire checks the number you chose for X, not the amount of mana you actually spent or the amount of damage it will actually deal.

Banefire can be targeted by spells that try to counter it regardless of what the value of X is. If X is 5 or more, those spells will still resolve, but the part of their effect that would counter Banefire won't do anything. Any other effects those spells have will work as normal.

If its target is illegal by the time Banefire tries to resolve, the spell isn't countered, but it doesn't resolve. The target won't be dealt damage.