In the end, the shamans decided to pool their collective psychic energies by reincarnating as a single soul in a single human body to create an individual they called "the New Man." The thousands of shamans, as one, took poison, and as one, they died, their souls flowing into the Immaterium in a rush of psychic power that overwhelmed those daemons who sought to feast upon it with a cleansing, purifying fire, a flame imperishable that became one soul out of many.
A year later the child who would become the Emperor was born in a Neolithic settlement of Anatolian herders and farmers.
That's deep, deep old fluff. Still theorized to essentially be the Emperor's origin story but I think the Shamanism has since been dropped. Although it's hard to say because current lore never tries to dig down that deep. GWS learned a while back that their universe was waaayyy more interesting when veiled in half-truths and mythology, then when they actually spelled the whole thing out.
There's all sorts of fluff that's since been purged because it was too explicit, and whacky. Like the Emperor having real, honest-to-Emperor sons, who were going to be sacrificed to create the Star Child or something, so it could fight the final battle against Chaos and win. (Essentially, the Emperor's origin story done over again, except instead of from many-to-one, it's one-to-many-to-one.) That particular plot line was murdered sometime after the Black Library started cranking out its 2nd generation of novels, although it's still engrained into the fanbase's collective psyche, much like Squats.
Incidentally, I'm still looking to buy a copy of "Space Marine" by Ian Watson, part of the first generation of Black Library publications. Rumored to be the grimmest, darkest and most violent portrayal of Space Marines ever penned. Which if you've read any Space Marine fiction should say something. Having read some of Watson's earlier stuff, I can believe it. His Space Marines are human to a fault. Like, they're all recruited teenage hiver gang scum. While becoming super human and getting the arsenal to match, they still approach the world like overactive teenagers with the bloodthirst. Very little of the typical stoic and honorable Space Marine, a lot of the adrenaline-filled murder. It's been said that novel is the reason we have the "idealized" Space Marines we do today....because the "realistic" take on them was so horrifying they couldn't sell it to kids.