The Legion's history begins with the descent of their Primarch Fulgrim to the resource-poor world of Chemos, where strict rationing and improvisation were necessary to sustain life. It was decided to spare the infant and when Fulgrim grew to manhood he set about reviving his dying world. Old technology was restored, lost areas reclaimed, and the people of Chemos were able to spend time on art and culture. Fulgrim took pride in this, as he harboured a love of perfection in all things.
When the Emperor first arrived on Chemos Fulgrim swore fealty to him instantly, and was placed at the head of his Legion, who were to be named the Emperor's Children in honour of their devotion to Humanity's ruler. An accident had destroyed nearly all of the Emperor's Children's gene-seed, and so the comparatively small force was placed under the command of Horus the Warmaster for a time. Horus and Fulgrim grew close to one another, with dire consequences for the Imperium.
When the Horus Heresy erupted, Fulgrim rushed to the Warmaster's side, attempting to reason with his old friend. Instead, Horus seduced him, playing upon his love of flawlessness to weaken Fulgrim's loyalty to the Emperor. With Fulgrim broken, it was a simple task for Horus to convince the Primarch to turn to Slaanesh, who promised perfection in all things.
The rot spread from Fulgrim to his brother Lord Commanders including Lucius the Eternal, then to company and squad leaders, and finally all but a bare handful of Marines followed Slaanesh rather than the Emperor. The remaining loyalists fought bravely on Istvaan III but were eventually overwhelmed. The Legion then gleefully proceeded to aid in the destruction of the arriving Loyalist Legions.
All trace of decency amongst the Emperor's Children had vanished by the time they partook in the Siege of Terra. While other Traitor Legions assaulted the Imperial Palace, the Emperor's Children embarked upon a spree of terror and gratification amongst the helpless citizenry of Earth. Billions of defenceless civilians were used as experimental subjects in the effort to create ever-more powerful stimulants and pleasure-inducing chemicals, or were simply killed to sate the bloodlust of the Legion.
When Horus was defeated, the Emperor's Children left a trail of depopulated worlds in their wake as they fled to the Eye of Terror. As their supply of slaves dwindled, they raided the other Traitor Legions for fresh meat, and in the end were crushed by their angry brethren into bands of hedonistic fanatics. As to Fulgrim himself, his fate is uncertain. Some believe he was killed, others maintain that he was elevated by Slaanesh to the rank of Daemon-Prince and rules over a planet of unending, unlimited pleasure. To this day, warbands of Emperor's Children seek this world, but obviously its existence has not been publicly revealed.