Drach'nyen,
Born from humanity's first murder, it was more like a demon than a sword—a weapon against people, much like the Sword of the Emperor was a weapon against demons. It was inextricably linked to the Emperor.
The origins of Old Man Yellow had long since become impossible to verify. Almost no one in the Imperium knew this little secret, but Hogg was one of the few who did.
As far as he knew, long, long ago, when humanity was still in its primitive infancy, many Shaman priests capable of communing with the Great Ocean had appeared on Terra.
Unlike the old frauds who danced around beneath bridges, these Shaman priests were the real deal. They were Farseers in their own right; calling them old Shamans was no exaggeration.
Then one day, the Shaman priests decided to try something flashy. At the time, humanity had yet to spread across the Galaxy, and these Psykers, representing the entire species, offered themselves up in a collective sacrifice. Through the womb of a mother, they created an insanely powerful psychic caveman: the Emperor as he was now.
Like the rising sun shattering the darkness, Old Man Yellow represented all of humanity's Psykers at the time in a ritual sense. That was why he had been terrifyingly powerful from birth, standing head and shoulders above the other cavemen.
With his enormous, powerful build and psychic might representing the sum total of humanity's Shamans, Old Man Yellow became the first generation of rebellious youth among humans. He basically embodied every Primarch's flaws—a walking embodiment of absurdity.
He had Russ's gluttonous recklessness, Ferrus's Darwinism, Fulgrim's fussiness, Dorn's stubbornness, Jonson's savagery, Little Pony's arrogance, and Perturabo's much-touted awkwardness...
For better or worse, you could see nearly every Primarch's strengths and weaknesses in the Emperor of that era.