The player to your left takes an extra turn after this one, in which you can see all cards he can and make all decisions for him.
Any cards he would gain on that turn, you gain instead; any cards of his that are trashed are set aside and returned to his discard pile at end of turn.
You are not taking a turn with the deck of the player to your left; that player is taking a turn, with you making the decisions and gaining the cards. This is a crucial difference to keep in mind when considering card interactions - the "you" in all cards still refers to the player being Possessed, not the player doing the Possessing. Possession has several pieces to it: - You can see the Possessed player's cards for the entire turn, which means you will see his next hand during Clean-up. You will also see any cards he is entitled to see due to card rules; for example, you can look at cards he has set aside with Native Village (from Seaside). You can count any cards he can count. - You make all decisions for the Possessed player, including what cards to play, decisions those cards provide, and what cards to buy. - Any cards the Possessed player would have gained in any way, you gain instead; this includes cards bought, as well as cards gained due to Actions. The cards you gain this way go to your discard pile, even if they would have gone to that player's hand or the top of his deck or somewhere else. You only gain cards he would have; you do not gain tokens he would have (for example from the Dominion: Seaside card Pirate Ship). - During the Possessed turn, whenever one of that player's cards is trashed, set it aside, and that player puts it into his discard pile at the end of the turn, after Clean-up. This counts as the card being trashed, so, for example, you could trash a Mining Village (from Dominion: Intrigue) and get the 2Coin. Getting those cards back at end of turn does not count as those cards being gained (so for example, you won't get them). Other players' cards that are trashed during that turn are not returned. - Cards passed with Masquerade (from Dominion: Intrigue) are not being gained or trashed, and so are passed normally. Cards returned to the Supply with Ambassador (from Dominion: Seaside) are also not being trashed, and so return to the Supply normally. - If you make another player play an Attack via Possession, that Attack will hit you like it would normally. If you want to use a Reaction in response to that Attack (such as Secret Chamber from Dominion: Intrigue), you would be the one revealing the Reaction, not the player being Possessed. - Possession causes an extra turn to be played, like the card Outpost does (from Dominion: Seaside). The extra turn happens only after this turn is completely over - you will have discarded everything and drawn your next hand. Outpost only prevents itself from giving a player two consecutive turns, it does not prevent other cards or the rules from doing so. So, for example, if you play Possession in a two-player game, then after the Possession turn, that player still gets his normal turn. If he played Outpost during that turn though, it would not give him an extra turn. If you play both Outpost and Possession in the same turn, the Outpost turn happens first. If you make someone play Outpost during a turn in which you Possessed them, that player will get the extra turn and make decisions during it and so forth, not you; if you make someone play Possession during a turn in which you Possessed them, that will make that player Possess the player to his left, rather than you getting to Possess anyone further. Possession turns (and other extra turns) do not count for the tiebreaker. Once the game ends, no further turns are played, including extra turns from Possession and Outpost. - Unlike Outpost, Possession is not a Duration card. It is discarded in the Clean-up phase of the turn you played it. - Possession is cumulative; if you play it twice in one turn, there will be two extra turns after this one.