This card was exclusive given when you bought the game (or made a crowdfunding pickup) at the SPIEL '25 fair in Essen.
The QR code on the card says:
Promo Goblin:
"When you play the Promo Goblin, set it and opposing unit of your choice aside. The two play cards together and do not participate in the next round. At the end of the round, return the opposing Goblin to where you got it and take the Promo Goblin back to your hand (and you can play it again if you pay its gold cost again). Note that Ogres are not Goblins and are too stupid to play cards."
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