If you cast a spell for another cost “rather than paying its mana cost,” such as an emerge cost, you can’t choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast it.
Herigast, Erupting Nullkite
If you sacrifice a creature with {X} in its mana cost, that X is 0.
The mana value of a creature spell with emerge isn’t affected by whether its emerge cost is paid. For example, if you cast Decimator of the Provinces for its emerge cost and sacrifice a creature whose mana value is 3, Decimator of the Provinces’s mana value remains 10.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Lucas Graciano
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