How do hidden cards work, and what happens to them when units leave the battlefield?

Ruling: To hide a card, you must control a battlefield, and the hidden card is placed at that battlefield. If there are no units at the battlefield where the hidden card is located, the hidden card is sent to the trash.

Nuances:

• You can prevent hidden cards from being played by countering them with cards like Defy or Wind Wall
• The rainbow symbol on cards means "recycle any rune"