When Falling Star is used to target a unit on the battlefield, can Retreat be used to save it, and when does targeting occur? Can the opponent then target units in the base instead?

Ruling: Targeting happens when Falling Star resolves and creates two reflexive "deal 3 damage" triggers that go on the chain. If you use Retreat in response to Falling Star itself (before it resolves), the triggers will target other available units when they're created. If you wait for Falling Star to resolve and the triggers to go on the chain with their targets locked in, then use Retreat, those triggers will resolve without effect since their target is gone.

Sequence:

• Player A plays Falling Star and passes priority
• Player B can respond with Retreat here (triggers not yet created, will target other units when created)
• OR Player B passes, Falling Star resolves
• Player A selects 2 targets simultaneously for the reflexive triggers
• Triggers go on the chain with locked targets
• Player A passes priority
• Player B can now use Retreat (triggers already targeted, will fizzle if target is removed)

Nuances:

• Both targets must be declared simultaneously even though damage resolves one at a time
• Falling Star doesn't require targeting opponent's units, so if no legal targets exist on opponent's side, caster must target their own units
• If you only control one unit and use Retreat in response to Falling Star (before it resolves), opponent will have to target their own units
• If you control multiple units, it's usually better to wait for triggers to be placed and targets declared, then save the important unit