This fact is represented in the Chaos Space Marine Army Rule: Dark Pacts ... Below is a list of your available Detachments, ...

Every Warhammer 40K Chaos army got an updated Codex/Index – here’s where they stand in the meta now.

Any hint of complicated gaming should be cast out of your mind, for at its throbbing heart, Chaos Legion is ... and your ever-willing legion of helpers against an army of aggressors.

an army of undead warriors at your command. The legion of chaos mentioned in the title comes around with a tap of the L1 button. A "Legion" is a helper creature who fights alongside Sieg.