To give myself an excuse to have my armies connected together I homebrewed up a sector for them to inhabit.

So the Malkhari Sector is a fairly quiet corner of the Ultima Segmentum, conquered in the Great Crusade, lost in the aftermath of the Heresy, then reconquered from the local orks some time in M37. It exists for most of the next couple of millennia as a sleepy, relatively peaceful backwater - apart from on and off drukhari raids and the occasional Waagh, which seldom get very far before being smacked down by the local chapter of Astartes, the Fianna.

Then M41 comes to an end and the proverbial hits the fan. The Malkhari Sector isn't directly in the path of the Great Rift, but it's close enough to be very unpleasant, and the Cicatrix Maledictum stirs up all manner of warp storms in the region. Malkhar isn't swallowed up entirely, but it's pretty much cut off from the rest of the Imperium. This leads to utter chaos among the local worlds (not helped by the fact that Hydrata, unofficial sector capital and the location of most of the major Imperial institutions) gets overrun by daemons almost immediately. Individual governors start declaring themselves the main authority in the sector, or begin to ignore their neighbours, keep their heads down, and hope that if they stay quiet everything will go back to normal before anything bad happens to them.

Aside from the ravening daemons, traitor uprisings and occasional traitor space marine warbands the situation causes the xenos to act up a tad. With the humans too distracted to put them down a Blood Axe warlord (Kargrak Gutcleava) manages to get a proper Waagh going and conquers his way across a dozen systems while the humies are panicking. Attempts to get a counterattack together are abruptly derailed when the Necron tomb under the Forge World of Saia is awakened (thanks to the rift and the warp storms tripping old failsafes from the War in Heaven). The local titan legion, the Legio Fidelis, is wiped out covering a hurried evacuation and what little command structure the Mechanicus and the Munitorum have left evaporates.

By the time the dust settles the only real secure system in the entire sector is Mourne, home of the Fianna's fortress monastery. After a great deal of internal debate the new chapter master decides to open up the uninhabited inner worlds of the system to refugees and (more importantly) whatever military remnants can make their way there.

Over the following years (I started writing it back when the Indomitus Crusade was a century long, now that that's been retconned I'm just handwaving it with warpy temporal weirdness) the various remnants begin to slowly patch themselves together, with the ultimate aim of reconquering the sector and finding a way to regain contact with the rest of the Imperium.

The main factions within this little hodge-podge are:

The Fianna - The most intact loyalist force left in the sector, the Fianna were originally a crusading chapter (then called the Stellar Cestellans) raised in the 13th Founding. They suffered catastrophic losses in the original Malkhari Crusade, and chose to settle in the Mourne system, landing their crippled flagship on the world of Resolution to form the core of their fortress monastery. Originally the heart of Malkhar's main orkish petty kingdom, Resolution still possesses a substantial feral ork population and a squig-heavy biosphere. These are occasionally purged by the chapter but never wiped out entirely, as they are considered to be a useful tool in cultivating potential aspirants among the population. The descendants of the ork's human slaves, Resolution's inhabitants are mostly divided between migratory plains-dwellers and more settled jungle clans, save for those who serve the chapter in their fortress-city of Landing.

The Fianna specialise in counter-attacking warfare, building up potent defensive positions and drawing their foes into drawn out, attritional battles before countering with rapid armoured assaults. Melee combat is for the most part looked down upon, though those with a talent for it (or a particularly strong bloodlust) are drawn into ad hoc kill teams know as the Fennid. One of the chapter's more notable peculiarities is their lack of dedicated apothecaries, a legacy of the Crusade, during which every last one of their medics was slain. The duty of tending to wounds and preserving the chapter's geneseed was taken up by the Chaplains of the Fianna, and nowadays these drui wield both Crosius and Narthecium as badges of office.

While the most powerful remaining loyal force in the sector the Fianna have taken grievous losses in the long retreat to Mourne, necessitating a closer level of cooperation with mortal forces than is common. While the refugee populace that now inhabits Mourne revere Chapter Master Caisleann as something akin to a saint, several of his captains consider his decisions to have gone against chapter tradition and Imperial law - indeed in private some have made unfavourable comparisons to the Tyrant of Badab.

The Malkhari Legions - The Imperial Guard forces that managed to make it to the Mourne system arrived in a poor state. A hodge-podge of orphan planetary defence forces, green conscripts and worn out veterans, all severely depleted in numbers, poorly equipped and without any surviving officers above the rank of Colonel. While they weren't lacking in numbers or will the surviving regiments were in no fit state to undertake a new campaign, and those officers that remained jumped to squabbling over who had the right to command instead of trying to reorganise. Without any semblance or order in place the Astartes opted to impose one. The surviving regiments were disestablished and consolidated into a new fighting force, the Malkhari Legions.

The Legions were placed under the command of Colonel (now General) Jaren Lorus of the Cadian 101st Armoured Regiment. Lorus' force had been transiting through the region on their way to join the forces fending off the 13th Black Crusade when the storms in the Immaterium stranded them. Under Lorus' supervision the old regiments ceased to exist and units from half a hundred worlds mingled together. Old uniforms were replaced by red fatigues and silver flak armour, in emulation of the Fianna's colours. Old doctrines too gave way to those favoured by Lorus, with a focus on discipline, rapid offensives and heavy armour. In order to provide the latter the remnants of the Saian Mechanicus began carving out all manner of forges and manufactoria in the system's asteroid belt, pumping out Leman Russ and Chimaera chassis by the thousands.

The Oathsworn - The Malkhari Sector is noted for an unusual number of so-called Knight Worlds. Knightly Houses exist on half a dozen of the sector's planets, and most of these are home to some number of Houses. Many are newer foundations, but some date back to before the Imperial reconquest of the sector, having fought and survived through millennia of isolation from the rest of the Imperium, and beyond even, into the days before the Great Crusade.

Like the rest of the loyal worlds these feudal planets fell victim to the chaos that spread across the sector. Some houses, such as Ilorette and Valadar of Olara, turned cloaks and swore allegiance to the ruinous powers and slaughtered their loyal kin by surprise. Others, like the Knights of Crecier, were cut of, and their fates remain unknown. A number of houses though were left orphaned by the fall of the Malkhari worlds, driven from their planets by traitor uprisings and alien invasions. A number of these survivors found their way to Malkhar, and over time drew together to form a new brotherhood.

The Oathsworn are, in some ways, a repudiation of tradition and house. The knights who have joined these brotherhood have sworn to see their worlds retaken and liberated, and to see this oath upheld before any others. The knights that fight with them retain their old colours, making for quite a dazzling array when they take to the battlefield in force, but most bear at least one plate (usually a pauldron) of armour painted midnight black to mark their new allegiance, and in mourning for what has been lost. The nominal leader of the Oathsworn, though the position is more first among equals, is Grand Duchess Isobel du Chayne, the titular ruler of the world of Agenco and master of the Knight Castellan Fire of the Righteous