Rule 702.118a

Emerge represents two static abilities that function while the spell with emerge is on the stack. “Emerge [cost]” means “You may cast this spell by paying [cost] and sacrificing a creature rather than paying its mana cost” and “If you chose to pay this spell’s emerge cost, its total cost is reduced by an amount of generic mana equal to the sacrificed creature’s converted mana cost.” Casting a spell using its emerge ability follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.

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