system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] The God-Emperor, as he is dying because of his wounds, is suddenly sent back to the Stellar Exodus with knowledge of what happens to Mankind,

What does he do to prevent the Age of Strife, the Horus Heresy, ect. ?
EDIT: Forgot to mention, he gets healed to perfect health when he arrives.
gpt
The answer to both questions is simple. Hoard STC's. If humanity never lost the Dark Age Tech, they wouldn't need to worry about troubles during the Age of Strife (since it wasn't really preventable) and there would be no need for Primarchs with the insane levels of tech the Imperium would have. 

The reason the Emperor never revealed himself was because humanity was doing just fine up until the AoS, thanks to their tech levels.