One of the many Space Marine Chapter Foundings since the Legions were created for the Great Crusade. Out of all the Foundings, the 13th Founding is the only one in which the Adeptus Terra doesn't possess any of the gene-seed from the founded Chapters. In what can only be called some of the worst record-keeping in the Imperium (which is no mean feat for the Administratum), the details of the Founding are all but lost; consequently, it is known as the Dark Founding.
Only three chapters have been confirmed to derive from this founding: the Exorcists, Death Spectres, and the Crimson Sabres (who were corrupted into the Crimson Slaughter). However, it is all but certain that numerous other Chapters were made at this time, but there are no records that can confirm exactly how many were formed, let alone give a list of their names and parent Chapters.