When using Hidden Blade's hide ability, does it have to kill a unit on the battlefield where it's hidden, and what happens to hidden cards if you lose control of that battlefield?

Ruling: When Hidden Blade is hidden, it must target a unit at the battlefield where it is hidden. If you lose control of the battlefield where a card is hidden, that hidden card is moved to the trash.

Nuances:

• All choices for a hidden card must be picked from among valid targets at the battlefield associated with that Facedown Zone
• You must control a battlefield to hide a card initially, and that battlefield cannot already have a facedown card hidden there