Second Founding: The Second Founding is the collective name for those Space Marine chapters created from the gene-seed of one of the First Founding chapters. Following the Horus Heresy, Robute Guilliman split the old Space Marine Legions into smaller Chapters. If a legion was too large to form a single chapter, even after the casualties of the war, then Second Founding chapters were made from a proportion of its members. The second foundings were designed to ensure that no chapter was so large as to pose a single threat to the Imperium, and also to give each an equal size.
Source: Codex: Space Marines, 3rd Edition by various
Submitter: IBBoard
Hive World Terra