When casting Falling Star, do you choose targets when you cast it or after it resolves? Specifically, if opponent uses Retreat in response to remove their creature, can you be forced to target your own unit?

Ruling: Falling Star uses Reflexive Triggers, so you choose targets after the spell resolves, not when you cast it. If your opponent uses Retreat in response to remove their creature before Falling Star resolves, you must choose valid targets from what remains in play when the triggers are created, which may force you to target your own unit if that's all that's available.

Sequence:

• Falling Star is cast
• Opponent plays Retreat in reaction (before Falling Star resolves)
• Retreat resolves, removing opponent's creature
• Falling Star resolves, creating two reflexive triggers
• You must select valid targets for the triggers from units currently in play