The Twenty Third Founding (also known as the Sentinel Founding) of Space Marine Chapters occurred sometime during the latter part of the 37th Millennium. Imperial scholars believe that the 23rd Founding to have been one of a series of linked Foundings around this tumultuous period in Imperial history, designed to repair the power and reach of the Imperium's Legion Astartes which had suffered considerable losses in the preceding millennia. According to the Requiem Malesent of Saint Kybra, no fewer than fifty-seven Space Marine Chapters had been destroyed outright, turned Renegade or declared as lost during this troubled period, which had seen calamities such as the horrors of 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's frontiers.
Several Chapters were, from the outset, conceived as Chapters that would spend their existence on continuous crusade and were to be created from the most stable gene-seed stocks available to the Adeptus Mechanicus at the time. 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, with the Marines Errant 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 at this time remains lost to posterity.
The Star Phantoms is another Chapter that was 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 primogenitor Chapter remained mysterious even during their creation, as did the backgrounds of many of the Chapters of this founding. Some sources have since hinted that they utilised 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, which along with the inherent introversion of character has given birth to a likely unfounded assumption by certain scholars of the Adeptus Terra and Deparmento Stratagos.
Select Twenty-Third Founding Successor Chapters
| Chapter | Descended from |
| Imperial Harbingers | Unknown |
| Marines Errant | Eagle Warriors |
| Star Phantoms | Dark Angels (Suspected) |
| Steel Cobras | Unknown |
Sources
- Imperial Armour Volume Nine - The Badab War - Part One (2010), 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