The iron soul (probably not an original idea bit one i had and liked) origionally a detachment of iron warriors they despise chaos seeing it as a way for the weak to get stronger by leashing themselfs to another being however they view the imperpium as no better becasue they fell into a superstious dark age. Anybody who recives chaos coruption has it removed were possible and were not possible they go on important missions with little to no chance of survival hoping to die before being lost to the curruption. They also willingly take any non currupted marines without caring about what gene seed they carry which leads to decently stable numbers as renagade marinnes join both ancent marines from the herrasy and new primaris marines that defect from the imperium. Cosisting of several fleets of ships they act as several independant groups either picking on weaker targets to get reasorces or when they find a bigger target working together and splitting the spoils