>>96065292
They're these dudes
https://wh40k.lexicanum.com/wiki/Serpentine_Scourge
who are only a warband because that's how Lexicanum decided to interpret the flavor text of picrel. Could simply be his nickname.
I just wanted green and silver CSM but didn't want the ebin ninja version of AL. Some anon gave me the idea.
I also play them as Tzeentch-aligned because I have always liked Tzeentch but I think TS look kinda ass. The idea of god-specific CSM warband is just appealing.
Lexicanum lists them as Undivided on this page
https://wh40k.lexicanum.com/wiki/Chaos_Space_Marine_Legions_and_Warbands_(List)
but they have no source for that so I'm not sure where it came from.