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