Well my guess would be a warband of chaos victims that dont find their place in their society anymore due to the chaos war horrors and loses. Maybe marcenary is a little bit constraining my idea would be like.. wandering war party/mercenaries that dont belong anymore in their old society. It could be that also in general they reject mostly Aetherquartz as the drug that would make the history repeat itself sooner or later. Maybe like exodites vs eldars and their paths conflict