Do I have to show the facedown card that was put in my hand with Pack of Wonders?

Ruling: Yes, you have to show the facedown card to your opponent.

Nuances:

• Since only cards with the Hidden keyword can be played facedown, opponents must be able to verify that any facedown card is indeed a card with Hidden
• This verification prevents illegal plays where players could bluff with non-Hidden cards played facedown
• Opponents must be given the opportunity to verify facedown cards when they move anywhere or at the end of the game
• If an opponent gathers their hidden cards quickly without allowing verification, they might be cheating