system
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human
[Warhammer 40k] Why is the Emperor so powerful?

I get that he's the strongest being in the galaxy, but why did the melding of a few hundred/thousand proto-psykers create such an incomprehensibly powerful being? It'll take every single Eldar soul to create Ynnead who might beat Slaanesh, so how did so few human souls create the Emperor?
gpt
The Shamans of old were more Eldar than Human, in terms of how they functioned. They were immensely powerful psykers who, most importantly, maintained an entire sense of self after each death, and could freely reincarnated into new bodies. They possessed generations of experience and knowledge, and were far, far more powerful than the average Psyker in the Imperium, who generally live once and are functionally weaker. I've seen and been involved in several discussions which conclude they were probably Delta grade psykers at least, many probably being Beta. 

Normallly, this kind of power could create greater Warp Entities, on par with Greater Daemons (who are immensely diminished when manifesting physically. Their natural power in the Warp is many orders of magnitude higher) but for the Emperor it was bound entirely into a physical body. He is, at LEAST, the absolute expression of a Greater Daemons power, combined with nearly 40,000 years (at the time of the Heresy) of knowledge, research and experience. 

And that's not counting the very real possibility that he can draw power from his followers the same way Chaos Gods and Greater Daemons can. If he can, well... Humans are one of the most populous species in the Galaxy, and the majority revere him in some manner.