I wrote some background for my Space Marine faction, if anyone is interested.

Ember
The planet of Ember was, before the great crusade, a barely inhabitable blasted rock in orbit around an orange giant star. Ember was rich in mineral wealth and mined extensively during the Dark Age of Technology, but by the end of Old Night had been largely abandoned and its survivors - hardy stock, descendents of the mining clans - had reverted to a feudal technocratic state. The surface of Ember was blasted by solar radiation, and so it’s people lived primarily in the abandoned mining tunnels, fighting internicine battles with shortswords and iron gauntlets.
The planet was brought to compliance remarkably efficiently early in the Great Crusade by the Iron Hands, who then returned some time later, lead by Ferrus Manus himself to defend it from an Ork Waaagh. The Orks were driven off the planet successfully, after a crucial battle in which Manus was saved from death at the hands of a gargant by a pyrrhic charge by the Emberian PDF. In return for this, Manus pledged the service of a single Iron Hand in perpetuity. This role was known as the Lord Steward, and though the first incumbent abandoned the position to join the scattered legions after the massacre at Istvaan V, the position was reinstated after the end of the heresy. For the next ten millennia the position of Lord Steward waxed and waned considerably in power and prestige, usually unrelatedly, with great heroes of the chapter acting as glorified totems, and disgraced junior marines acting as de facto planetary governors.

The Ember Lords
By the start of the Era Indomitus, Ember was a thriving mining world, with frequent mechanicus fleets carrying minerals and ores from its depths to the nearby forgeworlds. The population was booming, and so when the current Lord Steward, Terrek Rauth, made a petition to Guilleman, the old ties were reforged, and a torchbearing vessel appeared at Ember with a force of primaris Iron Hands hand-picked by Rauth himself. Rauth submitted to the Rubicon Primaris, and was promoted from assault sargeant to second captain of the newly-forged chapter in one fell swoop.
The Ember Lords follow the fighting traditions of the people of Ember, preferring close-quarters tunnel and city fights: a form of warfare that has also lent itself well to ship-to-ship boarding actions. They field primarily Intercessor and Gravis-armored troops, with heavy firepower carried by dreadnoughts and hellblasters. A unit nearly unique to them is the Ember Immortals - armed with powered shields and blades, they cleave to the fighting styles of the breacher cadres of the original Iron Hands legion.
The Ember Lords cleave approximately to the Codex Astartes, although they keep no regular Tenth Company, with small forces of Vanguard, scout marines, and the occasional speeder or flyer attached to each other company on an ad hoc basis, known as Firewalkers after the surface-dwelling mercenaries that scouted new routes for the feudal kings. Notable among them is Rauth’s second-in-command, Lieutenant Paris, seconded from the Fire Hawks to lead the nascent scout companies.
One thing I'm going to change is precisely how the Immortals work - or, at least, I want to see how the new sword-and-shield primaris marines work out before committing to the conversion I want to do with some Medusan Immortals. I might get some Nemesis Daemon Halberds and have them with Power Spear and shield.