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

From bits that I've read from a few sources it sounds like ancient shamen rallied together in order to reincarnate their collective as a single being. However, I don't understand how their fusion could create such a powerful being when I cannot imagine them being able to stack up to some of the psykers present in the 40k universe.

So is there something that I'm missing or has the Emperor's power simply been exaggerated by the secondary sources I've looked at? Thanks.
gpt
A unique combination of circumstances.

First; in the time of the fusion the Warp had no Chaos Gods; it was only beginning to become.. turbulent. The Old Ones were in full swing, as were their child races. The gathering dark in the Warp made it harder and harder for these Shaman to reincarnate; they were essentially immortal and had thousands of years of memory to draw on as well as having massively developed mental abilities.

Second, the warp really was pretty damn quiet. Imagine it (the difference in attention between them and current Psykers) as the difference between standing naked in the middle of times square at 12:30pm thrusting wildly and standing in your bathroom taking a shower, alone. The warp inhabitants are basically japanese tourists that really want to take a photo of your junk. A lot of stuff was possible then that isn't now because of the lack of tourists.

So, you take a bunch of psykers with no fear of the warp, thousands of years experience and then have them go "HEY GUYS LET'S HAVE THE MOST FREAKY THOUSAND-SOME EVER". They bugger off to the Warp, their souls merge and in the process cracks open up. Energy floods in, increasing the psychic potential of those souls before they finally merge with the whole. The whole sucks in energy and changes its birthplace; no longer a being of the material realm but wholly of the warp. It jumps up a few ranks; the difference between a powerful human psyker and a powerful daemon, multiplied a few thousand times.. boom! Emperor!

Or more specifically, Chaos God, but without the Chaos and the birth pangs because of the nature of his birth.

Oh, and it probably is slightly exaggerated; he's a transcendent psyker, sure. But an Orky Waaaaagh managed to suppress his power enough that he required rescuing from Horus once. It was a BIG Waaagh, sure, but still. Chances are his strength grew considerably over the course of the great crusade until his moment of 'triumph', scaring all four gods out of Horus.