The card is right now like this (thanks to Aidan Millow for the formulation):
Mirage
Action-Duration-0+
At the start of your turn play a copy of the card on the mirage mat.
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While this card is in play, it is in the supply.
When you buy this return all cards from the mirage mat to the supply and you may overpay. If you do, put an action card from the supply costing the ammout you overpaid onto the mirage mat. Either way, put mirage in play.
But as Aidan Millow and eHalcyon pointed out, it has a big problem. Pretty much no matter which card you choose, it is too powerfull to let anyone keep it, so it will basically just remove a gain or a buy from both players each turn.
It could maybe be fixed with some sort of negative effect every turn, but either the negative effect isn't bad enough or it's too bad, so you don't want to get the card in the first place.
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I would advice you not to read the rest of this post, but I'll leave it here in case anybody wants to know how it started.
I've had an idea for a card for a while, but I just can't get the wording right. I don't have a name for it, so right now I call it X.
The idea is:
There's only 1 copy of it in the supply.
When you buy it, you choose an action card in the supply and pay for it.
You put X in play when you gain it, and it stays there. In the beginning of every turn, you play it as the card you chose.
While X is in play, it counts as in the supply, so other players can gain it.
I think the best wording I've found so far is the following:
X, $?, Action, duration
When you're about to buy this or gain it another way, choose an action card in the supply that you can afford. The cost of this is the cost of that card.
When you gain this, put it in play.
In the beginning of every turn, play this as the card you chose.
While this is in play, it counts as in the supply.
Is there anything rule-wise wrong in that text? I'm especially thinking about when other players gains it from you.
I hope it all makes sense. Otherwise just ask.
I would also appreciate if anybody has a good suggestion for a name for it.
Strategy clarification:
If you choose a very good card, then the opponent will probably buy card X from you before you have a chance to use the good card.
If you choose a worse card, then your opponent maybe has something better to do, so you get to keep it and get a little benefit every turn.
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The following is the first wording I wrote in this post (I'll advice you not to read this):
X $0+ Action, duration
When you buy this, you may overpay for it.
If you do, choose an Action card in the supply costing the amount you overpaid, and put it in play.
In the beginning of each turn, untill this is gained again, play the card.
When this is gained again, return the card to the supply.
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When you gain this, put it in play. It stays in play.
While this is in play, it counts as in the supply.
Which is way too long.