My mind has been on the Soulpact and how it came about recently, so have some brainstorming. Soulpact starts in the Nova Commonwealth(A neutral party) and spreads via the repos to prevent its suppression as enemy propaganda in civilizations still fighting the interregnum. The military background is why they turned to martial arts to develop a tradition in the wake of the gift, and tombkeeping is to justify them choosing Necromancy. Them having shades of the Foreign Legion and the Night's Watch is to contribute to their fixation on redemption.
Before the Interregnum:
In the days before the Gift and the Interregnum, in the lands that would eventually become the Nova Commonwealth, there was an empire that needed to manage its ghost population. Genuine magical talents and exorcists were few, so they turned to tomb building to appease the ghosts of the dead. Those tombs needed keepers, for even in the days before the gift, there was still the occasional Necromancer who sought to exploit the dead, in addition to old fashioned graverobbers, along with the possibility of maddened ghosts finding their way out to terrorize the general public. And so the dead had company. Soldiers that failed at politics, disillusioned soldiers, fallen nobility, foreign deserters, criminal mages, people who committed manslaughter, few questions were asked, so long as they were willing to keep to themselves while manning the boundary between the living and the dead. After all, they would die and be kept by the next generation of keepers, troubling the living no further.
Between the ghosts of the dead and experienced living soldiers wanting to keep in practice, the martial skills of the Tombkeeper community were considerable.
During the Nova Commonwealth's Interregnum: Then Aum bestowed the Gift. People figured out how to stop dying. The Tombs stopped being a viable place to keep the defeated from troubling the rest of society. The Tombkeepers developed the Diamond of the Soul in its wake, and a demogogue arose who roused them against the emperor, successfully casting him down. In the aftermath, they got into wars with other states, revolutionary or otherwise. The horrors of magical suicide bombing, the greatest soldiers of the enemy being resurrected, and soul repositories to counter those shook warfare to the core, revolting Tombkeeper society.
Nova Commonwealth Sect to international movement: After the 12 great sects united the Nova Commonwealth beneath their banner, they looked outwards, they learned just how many hells actually exist, and they got into a large number of philosophical arguments, focusing on measurements of Utility and the nature of acceptable conduct, especially in wartime, in such a world. The various sides published their arguments, along with primers on their particular approaches to the Diamond of the Soul to Universal Repositories, seeking to take their dispute international. The Archive, as it always did, copied these arguments to the other six universal repositories, which as they found their way to new generations of idealists and disillusioned soldiers in other civilizations, led to the birth of the Soulpact. While a heavily fractious international movement, they fought for dignity and standards in war, the salvation of the damned, and an end to fates worse then death (Opinion on what constitutes a worthwhile cost to end them may vary) and continue to this day, 400 years after the interregnum began and into this period of relative peace.