I was Possessing my opponent. On the Possession turn, I bought Haunted Castle. I was not forced to place two cards on the deck from my normal hand. There was no top decking effect at all.
Game #3501373.
The card test states
QuoteWhen you gain this during your turn, gain a Gold, and each other player with 5 or more cards in hand puts 2 cards from their hand onto their deck.
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Thus, the behaviour was correct, and I hope you also did not get your gold? ;)
Ah, because the Possession turn is really my opponent's turn and he did not gain the card; I gained it instead?
Yes, that's right. From the game's perspective, it is the turn of the possessed player, you (possessor) are just the "puppet master" that pulls the strings. I know it does feels like it is your turn when you possess someone.