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human
[Warhammer 40k] Just how powerful were the Shamans?

So I know from If The Emperor Had a Text To Speech Device that Big E was born from all of Terra's ancient Shamans offing themselves to combine their powers because demons and such were killing them and fucking with people. But how powerful could they have been to both die from random demons but also create the Emperor? That is like a dozen tiers of power.
gpt
So there were thousands to tens of thousands of shamans partaking in the ritual, and each was powerful enough to reincarnate upon death. 

Granted this is back when the warp wasn't  in much turmoil, but the fact that they did so suggests that they could be eldar-level (beta+) psykers. 

The emergence of predatory daemons meant that they no longer could, and said daemons could've ranged anywhere from lesser to almost chaos god levels. 

The fact that they still reincarnated a few more times past this point shows that they were able to resist at least weaker daemons, supporting the theory that they were at least moderately powerful psykers.

And thus, the combination of so many powerful psykers (during a period of relative peace in the warp) allowed for a supremely powerful one to be born in the materium, with all of the knowledge and experience of those that came before.