If opponent plays Falling Star targeting my Kaisa, and I Recall my Kaisa leaving only opponent's Poro on board, is opponent forced to kill their own Poro?

Ruling: Yes, the opponent must target their own Poro. Falling Star's reflexive triggers must go on the chain, and when finalizing triggers you must choose among legal targets. If the only legal targets are your own units, you must target them.

Sequence:

• Falling Star resolves and creates 2 reflexive triggers
• When putting triggers on the chain, you must choose targets from among legal targets
• If only opponent's units remain, opponent must target their own units
• Both triggers can target the same unit (overkill is allowed)

Nuances:

• Deflect does not add additional cost when you target your own units with your own spells, so the Poro owner cannot refuse to pay to avoid targeting
• If both triggers target one unit and it dies from the first trigger, the second trigger does nothing (unless death is replaced by Zhonya, in which case the unit takes damage again)
• This is a timing interaction - you can pass priority and wait for opponent to choose targets for Falling Star triggers, then use Recall in response