The history of the Emperor's Children in the period that followed the defeat of the Traitor Legions at the Battle of Terra is largely obscured from Imperial scholars, for obvious reasons.
Perhaps the greatest mystery surrounds the fate of the Primarch Fulgrim himself, for it appears that he disappeared entirely.
An accident had befallen the nascent III Legion early in the development of its gene-stock, setting it far behind its fellows.
The Emperor's Children Legion now exists only as scattered, autonomous warbands located in the Eye of Terror who are dedicated to their own pursuit of corrupt, hedonistic and usually murderous pleasures.
The Chaos Space Marines of the Emperor's Children are known for possessing outlandish mutations and surgical alterations as "gifts" from Slaanesh which are designed to make them more "perfect" in the eyes of their twisted patron.
Within fifty standard years of his arrival on Chemos, Fulgrim had risen to become its leader, and it was his dream that the wonders of long lost ages might be rediscovered and rebuilt.
However, it was only when the Emperor came to his world that the Primarch could begin to fully realise his vision.
As his soul was slowly poisoned by the insidious taint of the Prince of Chaos, Fulgrim's passions grew ever darker and his drives ever more extreme.