If my opponent plays Star-Crossed and I respond by playing Sacrifice targeting my own unit with Deathknell, what is the resolution order?

Ruling: The Deathknell ability resolves first, then Sacrifice, then Star-Crossed. Because killing a friendly unit is a cost for Sacrifice (paid during finalization, not resolution), the Deathknell trigger is created and added to the chain as a pending item before Sacrifice becomes finalized.

Sequence:

• Opponent plays Star-Crossed (finalized on chain)
• You play Sacrifice in response (added as pending item)
• During Sacrifice's finalization, you pay the cost by killing your Deathknell unit
• The Deathknell trigger is immediately added to the chain as a pending item
• Sacrifice becomes finalized
• Deathknell trigger becomes finalized (now on top of chain)
• Chain resolves top-to-bottom: Deathknell → Sacrifice → Star-Crossed

Nuances:

• When Star-Crossed resolves, the unit it targeted is already in the trash, so that portion of the spell fails to find a legal target and is ignored.
• Costs are paid during finalization (Step 4 of playing a card), not during resolution.