Herigast, Erupting Nullkite

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.

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.”

Colored mana components of emerge costs can’t be reduced with emerge.