To cast a fused split spell, pay both of its mana costs. While the spell is on the stack, its mana value is the total amount of mana in both costs.
Give // Take
When a fused split spell resolves, follow the instructions of the left half first, then the instructions on the right half.
Some split cards with fuse have two halves that are both multicolored. That card is multicolored no matter which half is cast, or if both halves are cast. It's also multicolored while not on the stack.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Steve Prescott
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