Curtains' Call

Causing an opponent to lose the game after you've announced that you're casting a spell with undaunted and determined its total cost won't cause you to have to pay more mana.

Effects that reduce what you pay to cast a spell don't affect its mana value.

You can't cast Curtains' Call without two creatures to target. If one becomes an illegal target after you've cast Curtains' Call, the other will still be destroyed.