system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Horus defeats the Emperor flawlessly on the Vengeful Spirit. How does this effect everything?
gpt
If the Cabal is to be beleived (and they should, they accurately predicted the events that transpired with the Emperor's victory) then Horus is consumed by self loathing and leads a slow campaign of absolute genocide across the Imperium, eventually succeeding in wiping out humanity in its entirety. 

Without humans to sustain them, the Chaos Gods wither away, and are replaced by new, less violent deities. Orks spread through the galaxy before imploding in on themselves due to a lack of major enemy and are quickly cleaned up by emerging other races and the resurgent Eldar.