What happens to a hidden card placed on a conquered battlefield when the minion controlling that battlefield is killed by a spell?

Ruling: When you lose control of a battlefield, any hidden cards on it go to the trash. However, you can flip or play the hidden card in the last available reaction window before losing control.

Sequence:

• Opponent targets your minion with a spell
• You have a reaction window before the minion dies
• You can flip the hidden card (if it's a minion) or play it (if it's an action) during this window
• If you don't use the hidden card, it goes to the trash when your minion dies and you lose battlefield control

Nuances:

• You can play any hidden card type in response, even Hidden Blade targeting your own unit
• Only one hidden card is allowed per battlefield
• Once the minion dies, it's too late to flip the hidden card