IMO the most critical thing in delivering on this is foreshadowing. If the players feel like they should have known all along, then it's a good twist, if the players figure it out and switch sides, then it's also a good twist, but if the players get to the very end of a quest and then you suddenly reveal the truth it will feel like you've pulled it out of your ass and the players will be annoyed.
IMO the most critical thing in delivering on this is foreshadowing. If the players feel like they should have known all along, then it's a good twist, if the players figure it out and switch sides, then it's also a good twist, but if the players get to the very end of a quest and then you suddenly reveal the truth it will feel like you've pulled it out of your ass and the players will be annoyed.