- When you play this,
+1 Coin per unused Action you have (Action, not Action card).
(This is not in the Supply.)
- This is a Treasure worth 2 Coins, like Silver. You play it in your
Buy phase, like other Treasures. When you play it, you get an extra +1 Coin per unused Action you have. This means Actions, not Action cards. So for example if you play Farming Village (which gives you +2 Actions), then Diadem, Diadem will give you an extra +2 Coins, for 4 Coins total. If you play no Action cards at all on your turn, you will
have one unused Action, so you will get total from Diadem.
This is a Prize; see the Additional Rules.