I’m going to be starting a pretty short campaign here soon. I’ve mostly got the plot figured out, and when I was helping a player with character creation they asked if there was a good patron for their Warlock character. My initial thought was, “well the BBEG would be interesting.”
The players are just exploring a ruined castle/island that’s been twisted and it’s inhabitants warped due to exposure to a great old one- they’re trying to solve the problem and make the place habitable again.
The warlock’s patron is the BBEG, an eldritch god trying to force his way onto their plane. He’s currently held out of the plane by a magical seal that can’t be broken by him or any of his servants. So the Warlock player needs to- subtly- get the party all the way to the seal and manipulate them into breaking it. I did this specifically to avoid him murder hoboing his party members. They have different goals, but they still need each other.
If this goes badly, I figure the worst case scenario is that he gets slapped by the other 5 party members. So I’ve asked him to have at least an idea for a second character.
We had a campaign where something like that played out.
The warlock in that case wasn’t the traitor (although he was kind of evil and ruthless), but a paladin in our group was. He built his character to spy on and eventually assassinate the warlock, which made sense in the story. The paladin coordinated this with the GM, although not with the other players. Over the early campaign the warlock saved the paladins life on multiple occasions. The paladin felt super guilty and came clean to the warlock, who asked his patron to chain the paladin to him and make him his loyal servant. Again, GM coordinated with all involved and henceforth the paladin took psychic damage when he acted against the warlock, didn’t follow his express orders or strayed too far from his side. He also got a few cool abilities out of it, him being a warladin (or pallock?) now and all.
This made for a great story, but basically killed the game. It took away the agency of one player and we found no way to resolve it neatly. The warlock and warladin players made new characters and the dark duo became our BBEGs.
If you want to do this, have a clear and defined exit strategy. Make it clear to the traitor player that his character will be killed if no other solution is found during the campaign. In this scenario the experience of the rest of the group is more important because they did not consent to this.
If you want to do a cool double twist, you could do the following. If he reveals himself during the final showdown to be a traitor, have the BBEGs sidekick also be a traitor. The two basically switch sides, the player switches characters and continues to play with the former BBEGs sidekick.
I could see how it could ruin the overall campaign going forward, but I’m not concerned with that for two main reasons: this won’t be a particularly sandbox-y campaign, and it’s going to be pretty short.
The two ways this ends for the warlock player are: he fucks up and reveals himself too soon, at which point he either dies in the 5v1 confrontation that occurs, or gets away at which point his character will be an NPC going forward. Or he gets all the way to the end of the campaign, and either succeeds or fails to convince the rest of the party to release his patron.
The BBEG having a sidekick that turns to the good side in return is an excellent idea. I’ll definitely give that some thought.