The Liberators (aka Cain's Liberators, Liberators of the Father, Legion of Cain)
This peculiar formation of traitor astartes first started appearing shortly after the second Liberation Crusade of Chaos Warmaster Cain. Starting relativly small for their later size their exact origins are unknown to the Inquisition. Their uniformity suggest the defection of an entire chapter as a starting point but even the blackest files of the Inquisition do not serve up a plausible missing loyalist chapter - neither point intelligence analyses to a missing existing traitor astartes formation.
The battlecries and behavior are another oddity, "Break the Chain*", "Free the Father", "Tear the Shackles*", "Liberty or Salvation", as they concern themselves with liberation of the supressed and enslaved. Indeed, their strong views and opposition of such are of main drivers of inter-chaos conflict between Cain-aligned forces and those not.
*Both these cries are also common starting points for lenghty speeches concerning corruption and alleged mistreatment of the underclass by imperial authorities. As they are made by transhumans such speeches are usually censored by Inquisition. Unusually not because of heretic effects but due to plain, but effective, incitement to revolt.
So these are mostly Angron's Boys after the Panacea heals the Nails into submission. Cain encouraged their anti-slavery stance after realizing two things: 1. He was stuck with them and 2. most heretical activities that result in daemons manifesting are the result of slaves being sacrificed. By encouraging their strong disapproval of such things (Born out of the realization what the Nails did to them) Cain ensures that such ideas are nipped in the bud - that it also frequently leads to conflicts with nominal allies he reason away as such getting ahead of things. Relations would have broken down eventually anyway.
Unlike most traitor Astartes they have access to both excellent geneseed facilties (another attempt of sucking resources by Cain, same for gear) they also grow themselves by ambushing (primarly World Eater) Khornate Beserkers and feeding them Panacea until healed - or liberated as they see it.
Cain, out of a policy of ignorance is bliss, has not investigated as to what Father is meant by "Free the Father". He is aware that a common misconception is that these are former plague marines that think Nurgle has been corruped and needs to be bathed in Panacea - a prospect he suspect would be fun to arrange, but not realistic nor what is meant by the battlecry.
The astartes themselves have assigned a dual meaning, first to liberate their direct father and then the father mankind from their chains by (forcibly) healing them with Panacea. Yes, first Angron, then the Imperator.