If my opponent plays Falling Star and I have only 1 unit in play, and I use Retreat as a reaction to return that unit to my hand, does my opponent have to target their own unit with Falling Star's damage?

Ruling: Yes, if you Retreat your only unit in response to Falling Star, your opponent must target their own unit(s) with the damage triggers when Falling Star resolves.

Sequence:

• Falling Star is put on the chain without needing to target
• Retreat is played as a reaction to that link and resolves, returning your unit to hand
• Falling Star resolves, creating a chain with two links of "Deal 3 to a unit" that each need a target as they get put on the chain
• Since only your opponent's units remain in play, they must target their own units

Nuances:

• You can decline to pay deflect costs for reflexive triggered abilities (like Falling Star's damage), so if targeting an enemy unit with deflect, you can decline those triggers
• However, if you're forced to target your own units (because no enemy units exist), you cannot decline those triggers unless targeting your own units incurs a cost, which currently never happens