It is the 41st millenium.
He is by loving parents, if not unusually so for the standards of the time.
Also not unusually for the standards of his time, his uncle murders his father to take his property, and there is no force that would stop or avenge him. The boy is furious and distraught, and the murderer dies of a heart attack.
Later, he who will call himself the Emperor of Mankind, would say that this event first sent him onto the path of seeking to establish law; but that is a simplification that skips so many thousands of years and so many details that it might as well be a lie. The kid doesn't know what law is, he is illiterate and barely understands tradition; for now, he doesn't quite understand what he's done and why and if he in fact did that.
He leaves his tribe over distress, and wanders the wilderness, living off the land, wrestling with beasts and tribal warriors, making his first miracles with all the precision of a sledgehammer. It would be quite some time before he encounters what we now call "civilization".
Save for the occasional temporary friend he makes and an occasional vague vision he gets, he has no guidance in life. He slowly learns by experience.
By the time he is literate in a language, he has forgotten his first name, and there's nobody he could use it for.
He's lonely, but he doesn't seek much company; he doesn't know he should.
Gradually, he slots into civilized life.
A life in which he will walk many walks, from a family man to a playboy, from a soldier to a doctor, from a dictator to an artist, from a thief to a judge.
The "family man" role doesn't take. He's a loner at heart; he doesn't trust the idea of a family. Early trauma have shaped him quite a bit.
He's a loner at heart. He doesn't tell the whole world about himself. He educates himself and meditates. He sees ghosts of looking dangers that no one can; but they are subdued and he doesn't know exactly how real are they.
His deeds start a few myths, myths for which people will later kill and die.
It takes time, before any sort of moral philosophy, to conceive of the possibility of making things better as distinct from random acts of kindness. But our man of many names does possess a keen moral sense.
He doesn't take over the world; of course he leads a number of conquests, but it is simply as impossible to conquer a planet with Bronze Age technology as it is impossible to conquer a galaxy with steam engines and muskets.
He doesn't contribute to progress; progress isn't even a concept yet, although he will notice the trend earlier than most from his immortal vantage point. Mathematics is a curiosity and in infancy; craftsmanship is an imitative and personal skill; theoretical science isn't even a part of what philosophy there is.
He does sense and quench a few cults veering too close to Chaos. And he does, eventually, attempt to found a religion of his own, one that would preach law and unity while excluding worship of other gods. It goes well as a religion. It goes really quite poorly as a humanitarian intervention, especially in perspective.
And the times? They are a-changing!
The time is nigh for the Bronze Age to end.
This isn't going to end well, if Chaos is allowed to grow unchecked.
We must grow faster; we must grow stronger.
It is by this point that he who would later call himself the Emperor devotes himself to the cause of propelling humankind forward, and hope for humans to surpass the gods.
One would think that a pawn created to repel your sight and influence would be harder to manipulate to do your bidding.
So starts Antiquity, and in Greece humanity is invisibly reborn, and he who will later pronounce himself Master of Mankind is reborn with it as a guardian and cultivator of civilization.
It's 700s.
Peak of the Dark Ages in Europe. Few people know how to perform division.
The Middle East and Africa are not in what would later be called an Islamic Golden Age, instead being ruled by dogma.
In East Asia, print is invented, but fails to take hold.
(America is, as always, isolated, forgotten, and struggling with finding a use for wheels.)
Around the world, it is as if a shadow have obscured the light of progress.
But the immortal man of many names is, by now, already a man of immense skill and power.
And the Earth is far from defenseless.
If someone tried to turn it's hapless, ignorant, puny inhabitants into toys, thinking they were playing with their future food, they would be in for a surprise retaliation.
How is he out in space?! I can see every single bit of technology used on this miserable planet, and nevermind spaceships, they don't have powered ships!
I am the guy currently hitting you into your weak spot with a sharp slab of iron.
Fuck you, that's how.
How are you a native psyker?!?! I thought this planet didn't have any, didn't even know they were a thing!
I am a god! You can't just kill me, even after sneaking up on me using absolutely no technology I can feel and delivering horrible blows! You cant!