When casting Fallen Star or Icathian Rain, do you have to retarget your own units if your opponent uses Retreat to remove their units?

Ruling: It depends on when your opponent plays Retreat. Fallen Star and Icathian Rain create reflexive triggers that don't declare targets until after the spell itself resolves.

Sequence:

• The spell (Fallen Star/Rain) is added to the chain
• Opponent can react to the spell before triggers are created
• If no reaction, the spell resolves and creates damage triggers
• Targets are declared when triggers finalize onto the chain
• Opponent can react to the triggers after targets are declared

Nuances:

• If opponent reacts to the spell itself (before triggers finalize), you must retarget any remaining legal targets when the triggers are created, which can include your own units
• If opponent reacts to the triggers after they've finalized with declared targets, the targets cannot be redeclared and the triggers resolve with 0 effect
• If all units are removed before triggers finalize, the triggers fail to finalize and are not added to the chain (the spell still resolves and costs are paid)
• The spell does not return to your hand - it is discarded normally
• In tournament play, if you announce targets when casting, this is a shortcut that locks in those targets unless your opponent responds