I have a similar backstory for mine. They are kinda like Deathwatch except they for after Chaos Marines and they are made out of the geneseed descendants of Astartes who stayed loyal from every traitor legion. They use a lot of Terminator armour and Centurions and ignore the Codex Astartes so they have about 5,000 Battle Brothers, using more Astartes than any other Chapter for each task. And of course, they are fanatically loyal to The Imperium, bearing the names and some of the visual markers of their Legion on one shoulder out of shame but the rest of their armour is white and gold with tons of prayer scrolls and purity seals. This gives them their name: The White Retributors.





As an example of how they operate, a seven man Terminator squad sent onto a Chaos Space Hulk would consist of:



Iron Warriors or Luna Wolves Sergeant

Thousand Suns Librarian

Alpha Legion or Night Lords Assault Marine with storm bolter and chainfist

World Eaters Assault Marine with a pair of lightning claws

Death Guard Heavy Weapons Bearer with a heavy flamer

World Bearers Apothecary who also serves as a Chaplain

Emperor's Children Heavy Weapons Bearer with an assault cannon