Where do you hide Zhonya's - at a battlefield you occupy or in base?

Ruling: You must hide Zhonya's to a battlefield you control, not to base. When you play Zhonya's from hidden, you play it to base.

Nuances:

• Zhonya's can be played whenever you have priority (whenever you could play a reaction), not just in response to a unit dying
• You cannot play a hidden card on the same turn you hide it; you must wait until a subsequent turn