system
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human
[Warhammer 40k] What's the point of becoming a Demon Prince?

Most of the traitor Primarchs became demon princes, but you almost never see them actually do anything. As far I as I know, people who submit themselves to the Chaos Gods gain a massive amount of power, but the more tainted by the Warp you get, the less of an impact you can have on realspace. So then what's the point? You can be one of the most powerful beings in existence, but if all you can actually influence are a bunch of mindless demons and four 'Gods' that have realistically just dicked around for millennia, is there actually any point in it? Power's useless if you have no way to actually use it, after all.
gpt
Long story short is because its a lie. They eschewed slavery under the Emperor for slavery under someone who views them as little more than a trophy but this time they cannot rebel. The Primarchs to a (man, primarch, superior being?) well to the last one were all extremely ambitious, most kept their amibitions to the receipt of the Emperors favor others not so much. For Horus it came down to out right lies and a twisting of his understanding until he believed he was stopping a tyrant. The other traitor Primarchs followed this idea combined with differing degrees of desiring power or knowledge.

For others this same thought process is repeated, desiring power and immortality they sacrifice themselves in a self defeating proposition that takes away their free will putting them under the heel of whatever god they choose. Generally, nobody tells prospective Princes the whole truth, either because they stand to benefit from them doing so or they cant or dont know. We see stories of Princes sundering worlds with great effort and starting small empires in the name of their god. But we dont see beyond the veil to understand them as the tool they are for their god, the tortured souless being who is stuck upon the whims of another. We hear about the great effort needed to remove these undoubtedly powerful threats but none speak of the near inevitability of their disposal back to the warp and the eons they spend in that hell until called forth to do their gods bidding once more.