Does Fox Fire target units when cast, or does it target the battlefield and choose units on resolution?

Ruling: Fox Fire targets the units when put on the chain, not the battlefield. You must choose a legal set of targets (units with total Might 4 or less) when casting.

Sequence:

• When casting Fox Fire, target any number of units whose total Might adds up to 4 or less
• If targeted units gain Might as a reaction, choose a subset of the original targets with 4 or less total Might on resolution
• Some of the original targets become illegal, but you choose which ones to affect

Nuances:

• You cannot target all units on the battlefield and then choose on resolution; you must choose a legal set of targets initially
• If a new unit appears after targeting (like a hidden Teemo), it cannot be targeted by Fox Fire since targets are chosen when the spell is put on the chain
• If you targeted a unit with Deflect and paid the additional cost, but that unit becomes illegal on resolution, you do not get the Deflect cost refunded