The 23rd Founding of Space Marine Chapters, also known as the Sentinel Founding, occurred during the latter part of the 37th Millennium. Imperial scholars believe that the 23rd Founding was one of a series of linked Foundings that took place during this tumultuous period in Imperial history and that they were intended to repair the power and reach of the Adeptus Astartes that had suffered considerable losses in the preceding millennia. According to the Requiem Malesent of Saint Kybra, no fewer than 57 Space Marine Chapters had been destroyed, turned Renegade or declared lost in the Warp during this troubled period. The era had been marked by such calamities as the Age of Apostasy, the disastrous Cursed 21st Founding, the attacks of the Forces of Chaos and a rampage of Orks almost unchecked along the Imperium of Man's frontiers.
Several of the Space Marine Chapters Founded at this time were conceived as Chapters that would pursue perpetual Crusades against the Emperor's foes. They were created from the most stable gene-seed stocks available to the Adeptus Mechanicus. The Marines Errant is one such Chapter created during this time. Further details of this Chapter's origins can be found in the Mythos Angelica Mortis, the great work concerning the Space Marines of this age, which lists five such new-Founded Crusade Chapters and their blood-progenitors. The Marines Errant are recorded as having the Eagle Warriors Chapter as their immediate forebears. The Eagle Warriors' gene-seed was drawn from the Ultramarines' lineage, although why the Eagle Warriors in particular were singled out for the honour of a "named" Founding being drawn from their ranks remains lost to posterity.
The Star Phantoms were also created during this Founding to aid the Imperium's defences in numerous ill-starred and vulnerable areas of the galaxy. Their exact origins and progenitor Chapter remained mysterious even during their creation, as did the backgrounds of many of the Chapters of this Founding. Some Imperial sources have since hinted that the Star Phantoms were Founded utilising Dark Angels gene-seed, although this has been vehemently denied by the Dark Angels themselves, and the Star Phantoms consider such queries, even via official channels, as inherently presumptuous and cause for offence. Likely this speculation is simply based on some observed similarity in livery, iconography and trappings between the two Chapters, which has given birth to an assumption about the Star Phantoms' origins by certain scholars of the Adeptus Terra and Departmento Strategos.
Another Chapter created during this time was the Imperial Harbingers, but like their fellow Astartes of this Founding their exact progenitor Chapter remains unknown. The Steel Cobras were a formerly Loyalist Chapter of the Adeptus Astartes that was created during the 23rd Founding. Unfortunately, their blasphemous worship of the Emperor of Mankind as an animal totem prompted a puritanical Crusade against them by the Ecclesiarchy , led by a particularly bombastic Cardinal. Faced with such an assault by the Imperium, the Steel Cobras turned to the service of Chaos to gain the power needed to defend themselves.
Select 23rd Founding Chapters
|Marines Errant||Eagle Warriors|
|Star Phantoms||Dark Angels (Suspected)|
- Imperial Armour Volume Nine - The Badab War - Part One, pp. 74-81
- Warhammer 40,000 Rulebook (5th Edition)
- White Dwarf 101 (UK),"Index Astartes: The Badab Uprising," by Rick Priestley
- Cities of Death
- Imperial Armour Volume Nine - The Badab War - Part One
- Imperial Armour Volume Ten - The Badab War - Part Two