If I use Stupefy on a unit my opponent wants to kill to pay for Sacrifice's additional cost, can I prevent the unit from being killed?

Ruling: No. Costs, including additional costs, are paid before the spell becomes pending on the chain and before the opponent has priority to respond. By the time you can play Stupefy, the unit has already been killed and is in the trash.

Sequence:

• The player announces they are playing Sacrifice
• During the "Pay Costs" step, they kill a friendly Mighty unit (e.g., Ekko, Recurrent)
• If the killed unit has a Deathknell, it becomes pending on the chain
• The unit is placed in the trash, completing the cost payment
• Sacrifice is finalized and added to the chain
• Only now does the opponent receive priority to play reactions
• The killed unit is no longer a valid target for Stupefy as it is already in the trash

Nuances:

• If the killed unit has a Deathknell, it resolves before Sacrifice resolves on the chain
• A unit "is Mighty" as long as its Might is 5 or greater