If I cast Retreat in response to Falling Star, removing all my characters, and my opponent has characters, must my opponent target their own characters with Falling Star?

Ruling: Yes, if there are legal targets available, the player must choose from them, even if they are their own units.

Nuances:

• This applies when Retreat is cast in response to Falling Star, removing all of the casting player's characters before Falling Star resolves
• The opponent's own characters become the only legal targets available