The Twenty First Founding was the largest since the Second Founding. It took place sometime immediately before the Age of Apostasy, a time of Civil war which divided and almost destroyed the Imperium. The new Chapters were dogged by bad luck right from the start. Several disappeared mysteriously whilst in action or in warp space.
Every surviving Chapter of the founding is affected by spontaneous genetic mutation of its gene-seed. As a result the Chapters have gradually dwindled in size as their inability to raise and induct recruits means that battle casualties cannot be replaced. Worse still, some Chapters have developed genetic idiosyncrasies, mutations which strain the tolerance of the Inquisition and threaten the Chapter's survival. Few Chapters have suffered as ignominious an end as the Flame Falcons whose spontaneous and extreme physical corruption turned them into a race no longer human or sane. The Chapter was declared Excommunicate and driven from its home world of Lethe by the Grey Knights.