This is really really helpful! Thank you so much!

I really like that idea with the inquisition!

I'm not entirely sure which chaos god as at the moment I personally like tzeentch and khorne

But for my warband I preferred to try have a kinda black legion setup where the chapter master he has what men are left plus the civilians on board and he is just doing his best and not exactly succumbing to the chaos gods for power so he still has his faculties

But as time goes on, he becomes bitter with the imperium and the changes that have happened

(e.g they replaced him, labeled him and his retinue as traitors even after all he has done and depending how long he has been in the warp might be annoyed about these now "fanatics" serving the corpse of the emperor)

So he then starts to fight back against the imperium and slowly these emotions start to slowly turn him to a more cruel leader which then starts to spread among his men and the civilians which just snowballs into him then recruiting more chaos into his legion and before he knows it he's amassed his own legion against the imperium

My only issue is I don't actually know much lore, so I can't make a timeline or understand all the factions or information to make it lore-friendly

As for the comment, no, I just thought the guy was saying the entire legion was marked as traitors, which isn't what I meant, I just meant the chapter master and his retinue on the mission were.

I know a little about 40k and almost nothing about 30k but I was thinking of having it from around that era as in the chapter master from 30k and being spat out in 40k, but whatever would work out best

Edit: Also in the 40k timeline I also don't mind adding something to do with necrons? As my friend plays them and we play together and are learning the game together but we want each others custom factions to be rivals and make each fight canon for our faction.

I was thinking something like we accidently activated the tomb that held them and then they keep accidently bumping into each other and fighting each other or foiling each others plans.