Can a Hidden Zhonya's be used as a reaction when an opponent Charms your unit to their battlefield?

Ruling: Yes, you can react with Hidden Zhonya's to the Charm effect. Hidden cards gain the 'reaction' tag, and since Zhonya's doesn't target, its effect isn't restricted to the battlefield where it was hidden.

Sequence:

• React to the Charm before your unit moves
• Use the Hidden Zhonya's during this reaction window

Nuances:

• Zhonya's doesn't target, so it can affect units on any battlefield
• You must react to the Charm trigger before the unit actually moves to the other battlefield