Horus was the first Primarch to be rediscovered by the Emperor during the early Great Crusade. Because of this early discovery, Horus grew to be the strongest among the Primarchs as he had grown up from a child to an adult at the side of the Emperor. For a long time he was the only primarch to have been discovered. Friendship between the Emperor and Horus grew rapidly and the Emperor soon trusted him enough to give him command of the all of the forces of the Imperium. It is said that he saved the life of the Emperor during the Battle of Reilis when they were surprised. The Emperor repaid this debt when he killed a massive Ork who was threatening the Primarch.

When the Emperor first arrived on Chemos, Fulgrim swore fealty to him instantly, and was placed at the head of his Legion. From the Emperor, Fulgrim learned of the distant world of Terra, of the Great Crusade to reclaim the sundered galaxy, and of his own origins.

Travelling to Terra to meet his new Legion, Fulgrim learned that an accident had destroyed the majority of the gene-seed designated for his legion, and without their Primarch, replacing it was a slow and laborious process. Fulgrim came to address the two hundred Space Marines of his legion, and the words he spoke were said to inspire the Emperor so much that he named the legion the Emperor's Children, and allowed them to bear the sign of the Aquila, the double-headed eagle that was the Emperor's personal symbol, on their power armour.

Fulgrim was anxious to make his contribution to the Great Crusade, but the comparatively small size of his Legion meant that the Emperor's Children were placed under the command of Horus and his Luna Wolves. Horus and Fulgrim grew close to one another while pacifying the Eastern Fringe. Eventually, swelled by recruits from both Chemos and Terra, Fulgrim was soon able to lead a crusade of his own, bringing countless worlds into the light of the Imperium

When the primarchs were scattered to the ends of the galaxy, Perturabo landed on a planet named Olympia. He was discovered climbing the mountains below the city state of Lochos. The guards, having realised this was no normal child, brought him before the Dammekos, the Tyrant of Lochos. Dammekos was intrigued by the child and brought him into his house and treated him as family. Supposedly, Perturabo never trusted the Olympians and refused to return any affection given by Dammekos. Dammekos spent plenty of time with his new son, but never got anything in return. Many saw Perturabo as a cold and brooding child but when you consider that he had been thrown onto a world with no idea of his origins or purpose, this is perhaps a little harsh.

When the Emperor arrived Perturabo immediately submitted himself to the Emperor's mercy and ousted the Tyrant of Lochus. Dammekos is said to have spent his remaining few years gathering forces to attempt to retake his power. He failed but created a current of unrest which would be used later.

Kharn is the greatest of all the champions to Khorne's name, second only to Angron in power. He wields a chain-axe, known as Gorechild rimmed with the teeth of a Mica Dragon.

At the Battle of Skalathrax, Kharn became both legendary and infamous amongst his legion. With a unholy fervor, he slew in a mad-rage Emperor's Children and World Eaters alike, earning him the name The Betrayer, scattering the legion into individual warbands forever. He stalked the burning cities of Skalathrax like the bloody-avatar of Khorne himself, unstoppable and terrifying.

For a time afterwards he stalked the Eye of Terror as a bloody butcher, murdering all he came across for Khorne.

During the Thirteenth Black Crusade, however, he had a chance at something more. Now Kharn stalks the bloody streets and plains of Cadia, reaping a terrible harvest from the already sapped defenders of the planet.

Before the events of the Horus Heresy, Ahriman had shared his Primarch's obsession with arcane mysteries and had come to be keeper of the now mythical 'Book of Magnus', a tome of incalculable sorcerous power.

It may have been knowledge gleaned from that very tome with which Ahriman conjured his now infamous master spell. The result of the so called 'Rubric of Ahriman', while imperfect, is said to have satisfied Ahriman in its consequence. A veteran Thousand Son from before the coming of Magnus the Red, Ahriman's revulsion at the corruption of the Legion was so great that even the terrible price of reversing it was not too high.

Magnus was not of like mind, however. So great was his wrath when the cabal was discovered that the Primarch threatened to obliterate them utterly, but the very patron who had worked the mutations upon them in the first place was said to have intervened. Who can say what the most enigmatic and capricious of entities intended? In any event, the Daemon Prince stayed his hand, instead banishing Ahriman from the Planet of Sorcerers and condemning him to wander the Eye of Terror and beyond in a hopeless quest to understand the Chaos god Tzeentch.

As Horus was slain and the Emperor mortally wounded at the end of the Horus Heresy, Abaddon tore the Lightning Claw (the "Talon of Horus") from Horus's armour and retreated with the Traitor Legions to the Eye of Terror. Abaddon renamed the Sons of Horus as the Black Legion to expunge the name of Horus, who failed in his attempt to take over the Imperium. He carries the Daemon Sword Drach'nyen, which flays the souls from those it strikes. No other leader has been able to command the loyalty of all the nine Traitor Legions.

Since then he has led several raids and invasions against the Imperium of Man. He has waged 13 full-scale Black Crusades from the Eye of Terror; in the most recent assault, Abaddon managed to gain a foothold in the Cadian Gate, although he was unable to consolidate his gains and they have largely been snatched from his grasp by a vicious counterattack from the Imperial Navy. Thus far his ultimate victory has always been prevented.

Prior to the Horus Heresy Fabius Bile was the chief Apothecary of the Emperor's Children. He began to secretly experiment in ways to alter the geneseed at the instruction of the primarch Fulgrim with the intention of making the legion "more perfect". These experiments led to the first examples of Noise Marines among other things.

Bile is a Chaos Space Marine and was once the Lieutenant Commander of the Emperor's Children, accompanying them to the Battle of Terra. While there he didn't take part in the main battles on the planet, rather he performed horrific experiments on the population of the world and managed to create special drugs to make the Emperor's Children more succeptable to extreme stimulation, much as Slaanesh demands. He himself did not follow Slaanesh and distanced himself from his legion and instead devoted himself to reseach into the forbidden technology of the creation of Space Marines.

Eliphas came to Kronus to turn the planet into a Daemon World. He begun his conquest from the Demon Gate situated in the Demios Peninsula on the tropical south coast of Kronus.

Ygethmor has experienced a vision which has charted his ascension to coveted daemonhood - a ritual must be enacted on a mountain of corpses in the ruins of the highest Cathedral in Hive Euryales (Medusa V's capital) at the exact instant the Warp takes the planet. Predictably, a great deal of blood needs to be sacrificed for this.

Calas Typhon hailed from the Death Guard's eventual homeworld of Barbarus, and possessed the blood of the warlords that had ruled the planet before being defeated by Mortarion, the Death Guard Primarch. He also possessed formidable psychic powers...and had already sold his soul to the Dark Gods of Chaos. With their aid, Typhon became the First Captain of the Death Guard and commander of the battleship Terminus Est. When Mortarion joined Horus in the worship of Chaos, Typhon killed the Navigators who remained loyal to the Emperor and assured his Primarch that he could lead them to Terra. Instead, he led them into a trap - becalming the Death Guard fleet in the warp, adrift, helpless...and at the mercy of Chaos.