A Space Marine founding taking place in the early 42nd Millennium (8th Edition), the Ultima Founding is unique as all Ultima Founding chapters are entirely made up of Primaris Marines, the shiny true-scale NuMarines with the latest and greatest technology, which is what happened when Roboute Guilliman told Belisarius Cawl that normal Space Marines weren't cool enough.
While many of the first NuMarines reinforced existing chapters (with Guilliman's Ultramarines being the first, of course), Guilliman also ordered a host of brand new chapters (with Guilliman's Ultramarines also gaining a plurality of successors...) created out of whole cloth. These chapters are the Ultima Founding.
All of the original First Founding chapters were represented in these new chapters, with each chapter tracing their history like any other successor chapter. Yes, even the Space Wolves, who despite their disdain for the Codex are undoubtedly thankful for a Successor Chapter which hasn't devolved into mutants. While they seem less susceptible to psychic anomalies like the Black Rage, they do seem to be either immune or just extremely resistant to genetic flaws, such as the Wulfen. It is unknown if this is immunity or simple lack of time to manifest the conditions. Their fieldings during the Indomitus Crusade and those following it seem to show that their genetic anomalies play a part, but have a much harder time taking over Primaris. For example, a Space Wolf still needs to be careful not to get too into the heat of battle, or they lose themselves to bloodlust; though going as far as turning into a Wulfen has yet to be seen. Likewise, the Blood Angels probably have the same type of issue.
Ultima Founding chapters were mostly given homeworlds on the edge of the Great Rift to act as a bulwark against Chaos, although some were created to replace destroyed chapters and were given their empty Fortress Monasteries to inhabit.
Many of the Ultima Founding Chapters are apparently a first for how a chapter is built. Namely Guilliman had whole chapters created consisting of Marines with mixed ancestry, with Marines that were placed in the same chapter and squads with no regard for the origins of their geneseed. This appears to create better relations among Primaris Marines.
Games Workshop appears to be intentionally pushing players to create their own chapters this time, going so far as to include a coloring book style "design your own chapter" with the announcement of the Ultima Founding.
Specifically, the founding started on the last days of 999.M41 and first days of 000.M42, intermittently continued throughout the Indomitus Crusade as chapters were founded or refounded and ended with the Indomitus Crusade after the Battle of Raukos on 111.M42 as the final core of Unnumbered Sons veterans disbanded to form the last Chapters of the Ultima Founding.
Ultima Founding Traitor Legion Loyalists?
As mentioned in Dark Imperium, Cawl apparently either planned to or outright did create Ultima Founding chapters out of ALL the original first founding chapters -- including the Traitor Legions, as well as the two infamously Lost Legions, II and XI. It is unknown if they were finished before RG objected to them, or if they were created and subsequently purged. Guilliman personally suspects that Cawl probably had them made despite his objection. See also: Sons of the Phoenix.
List of Ultima Founding Chapters
- Knights of Thunder, mentioned in the Blood Angels codex as having had their chapter's name inscribed in many victory monuments throughout the Imperium of Man.
- Nemesors, mentioned in the Chaos Space Marines codex as the first encounter of Fabius Bile with the Primaris Marines.
- Silver Drakes, mentioned in the Tyranid codex as having been ordered by Girlyman to turn back Hive Fleet Ouroboris' advancement.
- Umbral Knights, mentioned in the Space Marines codex offhand as, like the Rift Stalkers, remaining on the planet they had been responsible for taking, to hold and consolidate.
- Valiant Blades, mentioned in the Imperial Knights codex as having received a plea for help from House Hawkshroud and answering it, despite it not being meant for them.
- Sons of the Phoenix (though they might be Fulgrim's sons as well)
- Castellans of the Rift
- Knights Cerulean
- Praetors of Ultramar
- Silver Templars
- Void Tridents