Interesting effect. For anyone that has played magic it's basically the morph mechanic.

Pay 2 gold to put a card face down into the shadows. Into the shadows is a new play area that isn't in your hand OR in play. You may play ONE card out of the shadows at the begining of ANY phase by paying it's printed gold cost.

Right now the most interesting card to me is..

Wharf Rats
1s (meaning it costs 1 gold to bring it out of the shadows)

2 strength
Military Icon

CANNOT BE KILLED!

when the rats come into play choose an opponent and give them control of the rats. When the controller of the rats wins a challenge (note- any challenge!) they must discard a card from the top of their deck.

Very weird! I don't know how playable but pretty interesting.

Bonus Agendas!!!
There is now a Martel & Greyjoy 'treaty' agenda. Ignore out of house penalties on either greyjoy or martel cards and your opponent needs 5 LESS power to win the game.

are there still agendas that make you go to 20 power in standard?  otherwise, i'm not sure if the Greyjoy/Martell treaty is worth it.
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somebody once wrote:

I have the hardest time remembering which marines are which, aside from smurfs and those red dudes.

Nope. All agendas are 15 power win totals.