Thanks to everyone who answered I was working on a story where a member of chaos forms a conjince and renownces chaos
Just to add a point from the chaos side, it's worth bearing in mind that Chaos isn't really a physical taint to be cleansed like an infection. People may be tricked into traitorous activities, or fall into chaos without realising that they're falling (e.g. Soul Drinkers), but most have either chosen to reject the Imperium and see chaos as a natural haven and ally, or have chosen to pursue one or more aspects of chaos for the power, enlightenment or understanding that it offers. So the question isn't really "can someone renounce chaos and return to the Imperium", as the answer is yes in principle. The important questions are why they would want to do this, and why the Imperium (not known for its forgiveness) would be willing to allow them to do so. Good luck with the story.
He started as a blood angel. The red thirst drove him to khorne. And someone did something he didn't agree with like killed a child or something which caused him to slaugter his warband and renounce chaos. And he was forgiven by a schemeing inquisitor. How does that sound as a base?
The taint of Chaos is manifold and each kind has to be taken into account separately. - You might have simply a preference for the promises power andsoforth that Chaos makes. A change of mind would be all that is needed to get back. - Bodily and psychic mutations. Contact with the Warp influences the personality and it leads to material changes of your body (mutations). Those mutations aren't necessarily a a gift bestowed upon you by a God of Chaos. They might simply be the result of an interaction between your mind (your secret desires and your personality-traits) and the warp and the result manifests as corporeal changes. (See: Talon of Horus) AFAIK not reversible, though there are cases where mutants served as loyal citizens of the Imperium. - Daemonic compact. A contract with a daemon is not merely an exchange of words but your life-energies are bonded on a psychic level. You are now part of the daemon and the daemon is part of you. Whether they like it or not, this bond is the very reason chaos daemons are loyal at all to a mortal master. (See: Talon of Horus) I guess, the first step would be tracking down the daemon and killing it. Not banishing it back into the Warp. Cutting it off from the Warp with the aid of an Untouchable (or sorcery) and killing it. I guess, the remaining taint is rather weak and can be easily exorcised.
