Can you play a hidden Zhonya's at one battlefield in response to someone killing your unit at another battlefield, forcing it to be recalled to base where it will see the unit die?

Ruling: Yes, you can play Zhonya's from hidden at one battlefield and it will affect units dying at other battlefields. However, you cannot play it "in response" to something being killed during damage assignment - you must play it from hidden before the showdown enters damage assignment (e.g., in response to a Falling Star or similar effect).

Sequence:

• Play Zhonya's from hidden before damage assignment (e.g., in response to another effect like Falling Star)
• Zhonya's either gets recalled immediately to base or is played directly to base
• Zhonya's at base can see and affect units dying at any battlefield

Nuances:

• Zhonya's is not limited to its battlefield since it does not target anything
• Hidden cards have reaction:0, so they can only be played in response to other effects, not during damage assignment
• There is ambiguity in the rules about whether Zhonya's goes to the battlefield and is immediately recalled, or is played directly to base when played from hidden, but either way it ends up at base
• Effects that don't target still affect everything at all locations