MTG Keyword Rules — Comprehensive Rules Glossary

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 mana value.” Casting a spell using its emerge ability follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.

Emerge from [quality] is a variant of emerge. “Emerge from [quality] [cost]” means “You may cast this spell by paying [cost] and sacrificing a [quality] permanent rather than paying its mana cost” and “If you pay this spell’s emerge cost, its total cost is reduced by an amount of generic mana equal to the sacrificed permanent’s mana value.”

You choose which permanent to sacrifice as you choose to pay a spell’s emerge cost (see rule 601.2b), and you sacrifice that permanent as you pay the total cost (see rule 601.2h).