Third Founding
"Brother-captain – I should tell you that Rogal Dorn has urged the High Lords to grant writs of succession to the Second Founding Chapters as well. Any, in fact, that have the veterans, gene-seed reserves and materiel to support them. Over a hundred have already been approved, with the same number again currently being assayed."
- – Chaplain Segas of the Ultramarines
The Third Founding of Space Marine Chapters was initiated by the Primarch Rogal Dorn in late M31 (860-900 years after the Second Founding)
It was particularly notable because it was genuinely the first time that the Space Marine ranks were given explicit permission to expand since the Great Crusade; the Second Founding was merely a reorganisation of the remaining manpower and materiel from the Legions that remained after the Horus Heresy and only loosely constituted a "founding" when compared to the tradition of new Chapters being drafted in later centuries. The Third Founding necessitated canvasing the existing chapters for spare materials and veterans to hand down to these new chapters as part of their initial founding, but it would have ultimately meant an expansion of Space Marines numbers, as well as the construction of new ships and wargear to equip them with.
Although the 21st Founding is considered the largest founding ever (at least until the Ultima Founding), the Third Founding is especially notable because the number of Space Marines Chapters virtually doubled in an extremely short space of time. Over one hundred writs of succession were issued as part of the Founding with one hundred more being considered, which when one also remembers that by the end of the 41st Millennium there were roughly a thousand chapters, the Third Founding constitutes a tremendous jump in the total Space Marine population whether that second hundred chapters were actually approved or not. (Also remember that many of the 21st Founding Chapters were wiped out due to unacceptable Gene-Seed flaws, which would counter any long term rise in the Space Marine population.)
It also marked the first time that these new "Successor" chapters would be given successors of their own, and thereby expanding the lines of descent for Gene-Seed bloodlines and diluting the influence the traditions and histories of the First Founding Legion would have over their descendants. Only nine writs of succession (ie: 9%) were granted to the Ultramarines directly, but considering that the Ultramarines had the greatest number of Second Founding successors one can assume the Sons of Guilliman were proportionally represented. But in any case, these new Chapters would have only a distant connection to their First Founding Legion progenitor; even though they might have been seconded officers from the First and Second founding chapters as part of their generation, these new Third Founding chapters would have no history rooted in the Great Crusade and the actions of their Primarchs, or the tragedies of the Horus Heresy, therefore they would have constituted a sort of "fresh" start which may have been an alien concept at the time.
It also caught some by surprise, hinting towards a mistakenly held belief that Guilliman's Second Founding was the be-all-end-all for the Space Marines. Roboute Guilliman had been indisposed for nearly nine-hundred years and his position of Lord Commander of the Imperium had passed into mortal hands. Even though the position was Rogal Dorn's during the Horus Heresy, Dorn apparently did not take back the position since the post was still referred to as "Lord Guilliman" well into M32, quite possibly due to a lingering anti-Astartes resentment left over from the Siege of Terra itself. It's interesting, and perhaps somewhat ironic that it was Rogal Dorn petitioning for a new Space Marine Founding, considering that his objections to the Second "Founding" had nearly brought the Imperium to a civil war, although this time he was clearly asking for "more soldiers" rather than disagreeing to split his men into smaller groups. Though it must have been awkward for him to petition the High Lords of Terra when they were led by a human going by the title of "Lord Guilliman".
Because the Founding was announced unexpectedly, it caused the Ultramarines to scramble to conceal the evidence of their Primarch's breach of the Codex Astartes when he left an extra company in place to cover up the completely separate issue of the Imperium Secundus. The company in question was rolled into its own Chapter, forever disconnecting itself from the identity and history of its parent legion.
The precedent set by the Third Founding was followed over five hundred years later by the Fourth Founding; initiated by the incumbent "Lord Guilliman": Maximus Thane (himself a son of Dorn) shortly after the War of the Beast in 546.M32.