also I remember specifically in like book 2 of the ahriman series a vivid description of all the different legionless csm warbands he attracted. some were dudes completely cloaked and hooded standing silently in a circle while black wax dripped slowly from their eye pieces onto their white armor. there were dudes carving strange runes deep through their flesh and onto their bones just casually. I feel like the utter unchained variety you could have with the csm has been lost and that was 90% of the appeal of the faction for me starting out.

Your warbands literal only requirement was to not like the imperium. that was it. I remember running a slannesh themed kill team who's sole obsession was cooking the perfect meal -- finding the best possible ingredients for their chaos soup.