The Space Marine Legions effectively ceased to exist after this act, succeeded by hundreds of new Chapters. The remaining loyalist Legions were broken up by Roboute Guilliman as per the new Codex Astartes. After the Battle of Terra, these fled into the Eye of Terror where many fragmented into disparate warbands that continue to plague the Imperium to this day.

In the Horus Heresy, nine of the eighteen remaining Legions sided with Warmaster Horus against the Emperor. However some point during the Great Crusade, two of the legions were erased from all records for unknown reasons. Gradually, the twenty lost Primarchs were reunified with their respective Legion and led them to further glories, where they would begin to establish unique identities for themselves.

ĭuring the subsequent Great Crusade, the twenty Legiones Astartes were the tip of the Emperor's spear in his conquest of much of the galaxy. However while the Gene-Seed was created by the Emperor, it was perfected and mass-produced by the Gene-Cults of Luna after they were pacified by the early Astartes. Each of the twenty Legions were created from gene-seed derived from a single Primarch, which were themselves maturing deep under the Himalayan Mountains before they were stolen away from the Forces of Chaos. The new Astartes had superior lifespans and discipline to the Thunder Warriors, and were intended by the Emperor to spearhead his coming Great Crusade. The Legiones Astartes were originally created by the Emperor and his gene-wrights in the closing days of the Unification Wars and the extinction of their predecessors, the Thunder Warriors.

Horus heresy book 7 inferno