Have you ever considered just telling your players what you want to do and see how they feel about it? A lot of people have rightly pointed out that many players would come out of this encounter unwilling to extend their trust to future NPCs. If that's something you would rather avoid, the best way I can think of is to just talk to them about it. It's possible that they might think it'll be a fun challenge and will buy into the whole scenario. And if they balk, then it's likely that they would have had a very bad time if it were sprung on them and you can avoid a whole lot of negative consequences for the game.