OK, now start from a regular deck with Estates and transform it into a deck with Shelters.
Turn 1: Draw 2c 3e. Buy Witherweaver.
...
I feel so relevant!
*ruins the fun* you aren't though, because WW can't gain necropolis. if an effect just says "gain card X" and X is not in the supply, nothing happens. that was the main joke.
I didn't see the FAQ...
*beats the dead fun*
I don't really agree with your explanation either (but obviously it is your card, so...) If I saw that on a real Dominion card (From "Dominion: Unglued") I would assume the FAQ would read:
You can gain any Dominion card that isn't in the supply (e.g. Spoils, Madman, Tournament Prizes). If it is a Dominion card and it isn't in the supply, you can gain it. This could be a card from the trash, a card from your Dominion box, a card from your opponents deck, one of the extra Provinces you aren't using because it is a 2 player game, ect...
I don't really understand where the "it's not in the supply, so you can't gain it" idea comes from anyway. I don't think that is in line with the rules as they work for other cards. I mean, Bandit Camp tells me to "Gain a Spoils from the Spoils pile", and Spoils is not in the Supply. Witherweaver tells me to gain a card that isn't in the Supply, so that has to at least include Spoils in a game where a card brings them into the game. If the card isn't in the game, well that is another question, but I would have gone with the hilarious card described in my fake FAQ above.
Edit: I guess you can definitely argue that a gain that isn't in the supply needs to have a "From" statement. I can give you that, so...
Witherweaver v2:
Gain any card that's not in the supply from wherever it is.
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When you put a card on top of your deck, you may reveal this from your hand. If you do, other Players will know that you have it in your hand.