This came up in a game I was playing today. I think my opponent and I both looked at the board and concluded that rushing to Possession wouldn't be as strong as Horse Traders / Duke, so we went that way, but due to a couple of factors including shuffle luck and buying Coppers more diligently, I ended up winning the Duchy split, so my opponent changed tactics. He went for Governor/Ambassador/Possession; never did really manage to pull it off before ending the game, but it left me wondering. Had he started out that way, rather than trying to win the Duchy race, or perhaps on a different board without Duke, wouldn't that be a really good strategy?
To be painfully obvious, the combo works like so: You start by using Governor/Ambassador to build up a solid deck with golds and stuff in order to buy Possession. You buy several Ambassadors and use them to give Ambassadors to your opponent [even if they avoided buying Ambassadors because they saw Possession on the board]. Then you take megaturns with Governor's +3 cards, ending in Possession on your opponent; the fact that they've drawn so many cards off your Governors means that it's likely their good & green cards will collide with the Ambassadors you've been sending them, so you can then use Ambassadors during their Possessed turn to pass the cards back to yourself. As an added bonus, the possessed turn "eats up" the bonus cards you give them off Governor's +3, leaving them with just a normal turn. Even with no +Action or +Buy on the board, it seems like you could easily cause a 4-Province (or 4-Colony) swing in "one" turn, by buying one on your turn and returning two on the possessed turn. If your opponent's deck has good money, you might even manage to buy one using their mega-hand after sending yourself their green cards, for a theoretical total swing of:
* +1 Province on your turn
* -2 Provinces to opponent during possessed turn (Ambassador)
* +1 Province to yourself during possessed turn (same Ambassador)
* +1 Province using opponent's money on possessed turn
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= 30-point swing (Province game)
Unfortunately, I haven't been able to test it out (doesn't seem easy to test in solitaire mode) but I'm really curious if anyone else has managed to get this to pay off. Is it too slow? Pipe dream? Or does it seem actually viable, given the availability of all 3 cards?