Event · M31–M32
Second Founding
The Second Founding was the reorganisation of the Space Marine Legions into Chapters, undertaken by the Imperium of Man after the Horus Heresy. It began in the opening years of the thirty-first millennium, while the Imperial Palace on Terra was still being rebuilt after the Siege of Terra, and its effects continued into the thirty-second. The cause was the Heresy itself: nine Legions had turned against the Emperor, and the survivors of the loyal Legions were left as very large, single-command forces answerable to one Primarch. The Imperium’s surviving leadership concluded that such concentrations of strength could not safely be preserved. The principal author of the reform is recorded as the Primarch Roboute Guilliman of the Ultramarines, whose treatise, the Codex Astartes, set out the new order. Under it, each Legion was divided into a number of independent Chapters, conventionally about a thousand battle-brothers organised in ten companies, with the gene-seed of the parent Legion apportioned among them. The Codex also prescribed doctrines for organisation, command and tactics, so that no single commander could again field a force on the scale of a Legion. Acceptance was uneven: the Ultramarines and many of their sister Legions adopted the division, but accounts differ on how fully some Legions, such as the Space Wolves and the Dark Angels, conformed, and later tradition holds that certain Chapters kept older customs in defiance of the Codex. The Imperial Fists, whose Primarch Rogal Dorn had defended Terra, contributed successor Chapters such as the Crimson Fists. The consequences shaped the Adeptus Astartes for the following ten millennia. The Chapter became the standard unit of the Space Marines, the Codex Astartes remained the foundational reference for Imperial military doctrine, and later foundings drew on the same gene-seed lineages to create further Chapters as need arose. The reform also fixed the Chapters’ lines of descent, which are still cited by their parent Legion, and it explains why the Imperium of the forty-first millennium fields many hundreds of Chapters rather than a small number of Legions. On the Orrery timeline the event is anchored to Terra at the close of the Heresy era, and connects the Horus Heresy, the Siege of Terra, and the later history of each loyal Legion and its successor Chapters.
See where Second Founding occurred on the galactic map in the 41st Millennium.