If my opponent uses Falling Star on my only unit while they also have a unit on their field and I use Retreat at reaction speed to save my unit, are they required to damage their own unit or does the spell miss?

Ruling: If you use Retreat in response to Falling Star before targets are chosen, your opponent must choose their own unit as the only valid target. Falling Star does not target on activation.

Sequence:

• Falling Star is played and goes on the chain
• There is a reaction window before targets are chosen
• If you use Retreat during this window, your unit is removed from play
• When Falling Star resolves, the opponent must choose targets (this is when deflect is checked)
• Since your unit is gone, their unit becomes the only valid target
• Two instances of 3 damage are put on the chain targeting their unit
• Each instance of damage creates its own reaction window before resolving

Nuances:

• You have two timings to react: before opponent chooses targets and after
• To force damage to the opponent's unit, you must react before they choose targets, not after
• The two instances of damage are separate items on the chain, each with their own reaction window