So I had an idea for a campaign that I like, but am unsure if it's fair to the players or not. Submitted for opinions/suggestions:

The acolytes are recruited and progress normally until some particular mission where they somehow meet another Inquisitor or cell of more advanced acolytes. They work together out of necessity or whatever comes convenient to me as GM at the time. After such time, before splitting ways, the other acolytes/ Inquisitor offers them a favor or service or something to make another contact. Encourage said contact through typical devious GM ways. After second contact, have acolyes or Inquisitor offer to induct PC cell into their sub Ordo (or whatever name I come up with for it). Having advanced they now have leave from their normal chain of command to pursue their own leads, and get some good leads on chaos cults from the other group, who seems to be too busy to follow up on all of them. These leads bring in big chatches and bring down major plots, and the relationship goes well for PC cell. Then, after awhile, PC cell gets info on cult that starts to bring them back around to the Inquisiton, eventually into direct conflict with their erstwhile benefactors when at the climax it is revealed the Inquisitor was really a cult leader infiltrating the Inqisition, and the acolytes were his main cult members. All are legitimate Inquisitorial agents in good standing, but are also double agents. Many of the cults the PC's eliminated were rivals to this group and with the PC's help, they have managed to imbezzle tons of money and material to fight against the Imperium.

Is it fair to the players to have them help out chaos cultists that they think are good acolytes that maybe they look up to before because they have a good reputation and are well liked. before their treachery is revealed, and even after many other would not believe the PC cell?

If you can pull it off it think it would be awesome, your players would have a personal vendetta with these guys which would be awesome too.

Hi,

of course it's fair, it's more than fair, it's good story-telling.

If you do it right it'll give your players a great experience that'll really hit them once they figure out what's going on.

If you can get away with it, I say all the more reason to do it.

Yeah good idea for a campaign there, think you work for the good guys, but turns out you were used as a tool by the enemy you're supposed to be hunting down.

Perhaps have those traitor acolyte/Inquisitor put a bounty of the player's head, and now it turns into a 'The fugitive' type mission where they must prove the Inquisitor/acolytes are really chaos worshippers all the while staying one step ahead from an Inquisitor/Arbitrator/Storm trooper strike force chasing after them.

Yes, it is fair. It is the classic "working for the bad guy all along" plot.

I did one of these recently.. In my changes to the Illumination adventure, I had the players work for the Crow Father under the belief that he was an 'ancient god of prophecy' while he had them unknowingly eliminate his competition. (Ended with a TPK - but was still enjoyed)

If you're going to do this type of plot, the one way to make it fair is to take some time preparing clues that hint at the Inquisitor's(and his cell's) true nature. Some clues that are most likely going to be overlooked by your players is key if you absolutely don't want your plot thrown off its hinges. Otherwise, just let the clues fall in naturally where they would, and let the players do as they will. It should yield a pleasant result as the players get that "OHHHHHH" moment when all of the clues they didn't pay attention to before all add up to what they should have already figured out. I think it takes a bit of knowledge of how to measure the difficulty of clues for puzzles and investigations, while also knowing your players and their play-styles.

Ultimately, it's a strong story, and you should definitely keep the plot. Just throw in some clues that won't de-rail the plot but will make it all fair in the end.

I think it's awesome!

The only way it's unfair to the players, is if you prevent them from investigating this other group/Inquisitor. If the players get a bad feeling about what's going on, they should be allowed to start snooping around. They're trained investigators, after all, and even the mighty (in this case, their 'benefactor') may fall from on high.

I'm not suggesting that you deliberately give them clues (the bads should be too smart for that) but don't block them off from legitimate avenues of investigation, either.