Thanks for the posts...great ideas!
I'd like to try and keep them loyal...perhaps they have an unwavering belief in the Emperor's Truth?
I quite like the background of the Relictors - they may still be fighting in the name of the Emperor even after being declared excommunicate.
I thought the dynamic could be something like:
During a protracted campaign on a cultist planet, six companies of the Shadow Legion are seduced by the powers of Chaos. When the Chapter Master arrives with a relief force, he finds his once loyal marines in league with the cultists.
Despite having significant numbers, the cultists and traitor marines are gradually pushed back until, sensing defeat, the traitors make an escape in their waiting ships.
Having heard of this treachery, the Inquisition (unnecessarily suspicious of Chapter Master Ivanov's positive opinions on the Imperial Truth) act quickly and lay waste to the Shadow Legion's homeworld (a scarcely populated world).
Back at the planet, in the aftermath of the battle, word reaches Chapter Master Ivanov of the Inquisition's actions - feeling the shame of his former brothers' betrayals and not wanting to fight those loyal to the Imperium, he heads after his traitorous brethren towards their last recorded position.
Now excommunicate, loyal Chapter Master Ivanov decided the only way he can repent for the sins of his fallen brothers would be to hunt them down and kill them for their betrayal...destroying anyone that gets in his way!
In a nutshell...
*Edit for clarification*