system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What happens to a primarchs soul after they die?

I was reading up on some background lore for 40k and it mentioned that when the emperor destroyed Horus's soul it meant he couldn't be reborn so to speak in the warp. Assuming this is the case does this mean Ferrus Manus, Sanguinious, Konrad Curze etc are all out in the warp somewhere? Does this mean they could potentially return to the materium and kick some ass one day?
gpt
>when the emperor destroyed Horus's soul it meant he couldn't be reborn so to speak in the warp. Assuming this is the case does this mean Ferrus Manus, Sanguinious, Konrad Curze etc are all out in the warp somewhere?

At that point, Horus was a Daemon Prince. The reason it's worth mentioning that his soul was destroyed is because of this; his physical body might have been nearly indestructible, but destroying *that* is the easy part with a Daemon Prince, and it doesn't mean much. If you killed Horus at Terra by simply ripping off his head or nuking the ship, he'd be leading the Black Crusades today. For reference, look at Angron, whose physical body has been destroyed several times now and is no worse for wear. 

The other Primarchs don't play by the same rules because they are Primarchs, beings of the materium with large presences in the immaterium, not Daemon Princes which are fundamentally beings of the warp that happen to be wearing partially-human skin at the moment. When they die, they actually *die*. Their soul enters the warp and they are eventually torn apart by its horrors, just like everyone else in  40k. 

That said, there probably *are* several Primarchs still alive in the Warp, if only because several of them appear to have gone into it willingly at one point or another and since they did so with their bodies and the only things that can actually hurt one in a fight are other Primarchs, it wouldn't be too weird if Leman or The Khan or someone was still kicking around inside the eye of terror, half-mad after ten thousand years of unending combat.