If Zhonya is played as a hidden card at battlefield 1, then activated as a reaction to Charm moving the only unit away from that battlefield, does Zhonya remain in play even though there are no units left at battlefield 1?

Ruling: Yes, Zhonya remains in play as a gear waiting for the first unit to be killed, even though there are no units at the battlefield where it was originally hidden.

Sequence:

• Zhonya is hidden at battlefield 1 with one unit present
• Opponent plays Charm to move the unit from battlefield 1 to battlefield 2
• A chain opens and you react by playing Zhonya from hidden
• Zhonya (as a gear) goes to base waiting for the first unit to be killed
• The chain ends
• Charm resolves
• No units remain at battlefield 1, but Zhonya is still in play

Nuances:

• Zhonya does not target, so it can save a unit at any battlefield regardless of where it was originally hidden
• The "here" restriction on hidden cards is a targeting restriction that only applies to effects that target
• Effects that don't target aren't restricted by the location where the hidden card was played