Can a hidden unit card remain facedown and hold a battlefield when all face-up units died to a spell?

Ruling: No, a hidden card cannot hold a battlefield. If you ever lose control of a battlefield, any hidden cards there are immediately sent to the trash.

Sequence:

• When a spell targets your units, you have a chance to react with hidden cards by flipping them
• If the hidden card does not flip in reaction and the chain resolves with none of your units on the battlefield, the hidden card is trashed
• Hidden cards are trashed immediately once you lose control, not only at the end of the showdown

Nuances:

• You should have an opportunity to react with hidden cards before they are trashed