system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] Chaos marines and death and other things.

Does chaos marines truly die in battle, or are they reborn in the warp like demons? Or is that exclusive to people who have achived demonhood? 

If they premently die, how do they substain their numbers? I know that every leagion started at about 100 000 strong, but 10 000 years of war must have taken its toll. I've also heard that they sometimes steal geneseed, is this enough to substain their numbers or do their own geneseed still work? Even if its corrupted, aint that what they are aiming for, corrupted marines? 

Also, have any traitor leagion been totally erradicated, had their numbers reduced to zero, or to few to come back?
gpt
So, here are some answers.

As far as replenishment of numbers is concerned, there are a few different methods. There's the daemonculaba, there are Dark Mechanicus/Hereteks to assist with creation (remember, they're unbound by typical Mechanicus rules), there are plain old traitors and mercenaries, and there's... well... the nature of Chaos itself. Time is all wonky for them... from their perspective it could have only been a few years since they turned traitor, or it could be a million years. Chaos doesn't play by the rules of physical reality.

As far as death goes, their souls would return to the warp. Unless they're remarkably willful and powerful, their soul would just sort of "unravel" and become part of the warp rather than its own entity.