Nemesis of mortals casting cost

[Hu Gibbet011]

I was think about responses to a late game {nemesis of mortals} with a grixis control deck. If nemesis is cast for a reduced cost, and in response I cast crypt incursion to exile all creature cards from the caster's graveyard, would that increase the casting cost of the nemesis on the stack, or would it resolve for its reduced casting cost?

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-Keyeto

[FustyDavorite]

601.2e: The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can't be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes "locked in." If effects would change the total cost after this time, they have no effect.

Costs for the spells are paid before you would receive priority to be able to affect the spells casting cost. Once the costs are paid, it's done.

[MuggyWuggy]

You would essentially have to know that's in his hand and then crypt incursion before he cast the nemesis