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The number of targets you choose for Comet Storm is one more than the number of times it's kicked. First you declare how many times you're going to kick the spell (at the same time you declare the value of X), then you choose the targets accordingly, then you pay the costs. No player can respond between the time you declare how many times you'll kick the spell and the time you choose the targets.

Each target you choose must be different.

For example, if you want Comet Storm to deal 4 damage to each of three different targets, that means X is 4 and you're kicking the spell twice. You'll pay a mana cost of {4}{R}{R}, plus a kicker cost of {1}, plus another kicker cost of {1}, for a total of {6}{R}{R}.

As long as any of its targets are legal at the time Comet Storm resolves, Comet Storm will deal X damage to each of those legal targets.