Can you play hidden cards the same turn you hide them?

Ruling: You cannot play hidden cards the same turn you hide them. You can only play them beginning on the next turn at reaction speed, ignoring their base cost.

Sequence:

• When you hide a card, it goes facedown to the battlefield
• During the turn you hide it, you cannot play it
• Beginning on the next turn, the card gains [Reaction] and you may play it, ignoring its base cost

Nuances:

• By default, you can only play cards from your hand or champion zone, not facedown cards
• The Hidden ability grants permission to play the facedown card, but only starting the turn after you hide it
• [Reaction] means you can use it any time you would be able to use a reaction spell/ability (your turn, during a showdown, or after someone plays a spell or activates an effect)
• You can play the card from hand for its full cost to get the effect immediately instead of hiding it