[Lord Commander Erus]

The Planetary Defense Force of Sentarus was, before the Chapter came, actually made up of Skitarri and Tech Guard who served the Tech-Priests on the Sentarus Forgeworld. Sentarus was famous for both exotic weapons (needle weapons, STC variant tanks, etc..) as well as a ruthless fighting force well above the normal strength for a Forgeworld, even one so large as it.

The Arch-Magos and his cabinet of Magos Colegio (heads of the Adeptus Mechanicus departments within the Forgeworld) were corrupted by an arcano-tech relic tainted by the warp. As the Black Saints were fleeing the destruction of the home world from the Chapter War (with roughly the 1st and 10th company intact, and smatterings of others), they came through the Sentarus system and caught the distress signals from the Planet..

The Chapter was already sorely in need of repairs, but along with their duty to help the Imperium, they realized if they could free this planet, they would be able to be re-supplied and armed, and be able to lick their wounds. For what better world is there to recover on than a world of martial pride boasting the largest Tech Guard and SKitarii regiments uin the sub-sector?

They sent the full might of the First Company in after exhausting their bombardment supplies and eliminating several key points of the enemy's strength. All told they lost nearly a fourth of the First Company, but liberated the planet in less than a day total.

Gratefully, the Tech-Priesthood pledged to re-supply the Chapter, though the Chapter Master Adonis thought their oath to only be for supplies, he found a full regiment of freshly recruited Tech-Guard at his command the next day, commissioned to be trained and made into Marines, scouts, serfs, or whatever he saw fit.

The notice with them also stated most of them had received minimal wetware implants, and were only fifteen years old standard, though intense genetic manipulation, physical augmentation, and hypno-indoctrination would make them the ideal soldiers for the Chapter.

So it was the 10th company was promoted and the skeletal structure of the Chapter began to rise from the ashes. Each decade the Tech-Priests sent a cull of the brightest children form Sentarus to the Chapter, and in return the Chapter sent tech-aspirants to be immersed in the ways of the Machine-God. So it were to continue, until the two organizations were so intertwined, they were as one mail-clad fist, fully ready to bring the Emperor and the Machine-Gods wrath down upon all those they faced.

And it has been a hundred years hence, with the Marines training and recruiting a full company from the world, and engaging in several small suppression and purge actions throughout the sector. Current fluff takes place as the Chapter finally sets off on it's first Crusade since the Sundering (Chapter war). A mission taken to track down and slaughter every last traitor from the Chapter