Hi everyone!

So, i have an idea. I wold like to let someone in the party to join the chaos cult, become a traitor. What do you think about this move? How it woul be bettr to roleplay it, an how we can do this avoiding big troubles with rules? Thanks for answers.

The biggest thing to consider isn't about the game itself; it is how your other players will react. Some groups don't deal with those kinds of shenanigans well, and you should discretely figure out what they'll say/think about it. One good option is to look up some of the stories on previous people who did it online, and use it as a conversation starter.

The next biggest is Corruption (and Insanity, but moreso Corruption). Your traitor PC better luck out on those rolls, or they're going to get spotted and purged.

Make sure that you and the player agree on what the player is trying to do. Something with more planning than going along with what they are doing before the moment where he switches to "lol, I kill party members". Have him working against the party for a while.

I'd suggest giving the traitor some goals that require him to keep the party alive while betraying them over a long period, not what the Inquisition wants. For example, the traitor is manipulating them to remove threats to his cults plan while ignoring his cult. For example, burning competing cults, the occasional framing of a loyalist so that the cult can replace him with their man.

Ideally the only time he fights the other PCs will be either:

- They have discovered his betrayal.

- They are too close to going after his cult and he doesn't think he can redirect them.

I'd suggest you secretly give him goals relating to what the cult wants and let him decide how to accomplish them. Probably some secret mini-sessions so he can roleplay important parts of his plans without the other players directly knowing about it. Don't let his plans go exactly as he describes them to. Introduce complications. You will have to figure out how much information about his secret activities reaches other PCs. Too much and he won't get anything done before being discovered. Too little and the other PCs will be very angry when he turns with no warning and/or they find out that they completely failed because of things that they had no idea were happening.

I did something similar playing an Alpha Legion infiltrator in Deathwatch. You definitely have to be mature and have it more about the story than player versus player.

In my case I was undercover so deep that I didn't even know my mission until it was revealed at the end and being an "old school" Alpha Legionnaire I wasn't completely against my squad mates (our pledges to the Emperor were easy for me to make ). Even when revealed I chose to make off with the alien tech and not kill my squad mates (who were vulnerable at the time) as I figured it would be unfair to kill them suddenly when they had no warning what was coming. This did result in me being killed by the recovered PCs as I was being lifted into a waiting Chaos vessel, thanks in a large part to the CSM covering me having his heavy autocannon jam first time he tried to fire it and not being able to unjam it (but my body managed to make it in with the loot).

Because it was handled as a story arc everyone loved the twist at the and had a good time congratulating the GM and I on pulling it off.

I think Bilaterlope's advice is very good when looking at doing this.

Having the cultist's agenda needing the group to operate and survive helps to avoid conflict. Perhaps one way of doing this is have the cultist try to direct his group to rival cults thus leaving his own cult free to go about their nefarious business as well as eliminating rivals.

Also you could create two versions of the character, one with DH2 and one with BC and if he's ever revealed have him pull out his BC character sheet for the final showdown (this is what I did in my game). This also avoids the why doesn't he have malignancies and mutations issue if he's so corrupted as it's revealed he's crossed the 100CP threshold already which usually means having no mutations for newly created characters.

Good luck with the game I hope it's fun and enjoyable for all involved!

Juggling loyalties of the PCs and the cult is going to be harder than just being a loyal PC. I will warn the player about that when he suggests being a traitor.

I think what I'd do for a traitor is give him secret goals. Some would be orders from his cult, others would be along the lines of "you realize that your cult would like it if you do x". Or worse, "the cult would not like y happening". Fail too many of those goals, or fail a really important one, and the cult would worry about his loyalty/usefulness to them. Which means coming after him because the cult knows he is Inquisition, which makes him a serious risk to the cult.

Worse still is when a situation gets complicated. For example, PCs get into a fight with some NPCs. I then pass a note to the traitor player saying that he recognises one of his opponents as being a member of his cult*. Someone that might recognise him. Leaving him with bad options:

- Have fellow cult member die, maybe by his hand. If his cult finds out, he'd have to try and explain it to them. If the cult gives him a chance to explain.

- Have the fellow cult member escape. Which he might need to explain to the rest of the party.

- Have the fellow cult member survive the fight. Which risks interrogation blowing his cover.

*If the cult has infiltrated an acolyte cell, they are going to have infiltrated other groups.