Bilateralrope said:
The Eldar gave birth to a single Chaos God, yes (though, given the timeless nature of the Warp, Slaanesh could technically be regarded as having already existed before then, and may well have contributed to its own birth)...
...but Mankind gave the Chaos Gods an army, and one unlike anything that had existed before, legions of warriors totally one half of the armed forces of a civilisation that had conquered well over a million worlds in only two hundred and fifty years, led by superhuman beings whose individual might eclipses all but the most powerful of Greater Daemons.
The Horus Heresy is very much an ascendancy for Chaos, to the point where it has the means to directly influence the material universe through countless trillions of worshippers and warriors.