Rule 702.153a

Casualty is a keyword that represents two abilities. The first is a static ability that functions while the spell with casualty is on the stack. The second is a triggered ability that functions while the spell with casualty is on the stack. Casualty N means “As an additional cost to cast this spell, you may sacrifice a creature with power N or greater,” and “When you cast this spell, if a casualty cost was paid for it, copy it. If the spell has any targets, you may choose new targets for the copy.” Paying a spell’s casualty cost follows the rules for paying additional costs in rules 601.2b and 601.2f–h.

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