So, here's a bit of unexplored mechanic space: cards that mandatorily bottom-deck themselves. We see a little bit of this kind of thing with Stash, Star Chart, Secret Passage, or
Neirai's Shipwreck, but I think it could be explored more.
Cavalry - Action, $5
+3 Cards
+1 Action
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When you shuffle this into your deck, put it at the bottom.
A strong engine card, but by the time you get to the bottom of your deck, do you even need it anymore? You can either accept that your draw is unreliable and spiky, or you can find a trick to get it in hand. Pearl Diver OP. Not sure if it should cost $5 or $6.
Spelunker - Action, Attack $4
Trash a card. Per $ it costs, draw a card.
Each other player with 5 or more cards in hand puts one on the bottom of their deck.
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When you shuffle this into your deck, put it at the bottom.
A terminal variant of Research/Apprentice. The attack is mean, but if your opponent also has a Spelunker, they can bottom-deck a card they want to trash, or perhaps a village.
