Can you play Star-Crossed in response to a Deathknell trigger to return the unit to hand and prevent the Deathknell effect?

Ruling: No. By the time a Deathknell trigger is placed on the chain, the unit has already been killed and sent to the trash. Star-Crossed requires you to target units on the board, so a unit in the trash is an illegal target.

Sequence: To prevent a Deathknell effect with Star-Crossed, you must:

• Play Star-Crossed in response to whatever spell or ability is attempting to kill the Deathknell unit, while it is still on the board
• Star-Crossed resolves first (LIFO), returning the unit to hand
• The kill effect resolves but cannot kill the unit (no longer on board)
• Since the unit never reached the trash, the Deathknell trigger condition is never met

Nuances:

• The Deathknell trigger condition specifically requires the permanent to be killed and sent to the trash
• Once the unit is in the trash, it cannot be targeted by Star-Crossed