I have rewritten the fluff for my Knight Houses and Imperial Guard regiment in the aftermath of 8th Edition, Adeptus Titanicus, Mechanicum (and its evolution into the Adeptus Mechanicus), Lord Commander Roboute Guiliman's return, and all the other stuff. So I'm going to combine my old material with the new one here.
The Draconis Coalition
The Draconis Coalition is a cluster of systems located at the far northeast segment of Segmentum Solar. A Coalition of over two hundred worlds, united under the rule of the Shogun (Generalissimo), who rules on behalf of the Emperor, the core system is known as the Draconis system, home and capital to the Shogun and the largest Knight Houses in the Coalition. House Yato, House Uesugi and House Takeda reside on Draconis III, the third world from the star Draconis. Further out is the forge world Draconis IV, where renegade Tech-priests and outcasts from the Adeptus Mechanicus seek refuge and gather. Many of these exiled Tech-priests are those who have been banished from Ryza, and as such, Draconis IV seek to reproduce the plasma technologies of Ryza. That is also the reason why the Astra Militarum regiment on Draconis III - which also serves as the Household Retinue and military armed forces for the Knightly Houses of Draconis III - are equiped with more plasma weaponry than most Imperial Guard forces. The mainstay of their armored regiments, the Draconian Armored Defenders, are Leman Russ Executioner tanks. Recently, they have been re-manufacturing Volkite Carnodon tanks that had been mothballed for ages inside the manufactorums in Draconis IV since the days of the Horus Heresy and Great Crusade, but these remain rare and eschewed for the more powerful and much sturdier Leman Russ Executioner tanks. Indeed, entire regiments of Draconian Armored Defenders consist of nothing but Leman Russ Executioner tanks, with perhaps an occasional Shadowsword as super-heavy support and titan-slaying. Otherwise it's mostly Leman Russ Executioner tanks and plasma-armed Draconian infantry, living up to Draconis IV's reputation as the Second Ryza.
Currently, the Shogun is Yato Yoshimoto, the equal of a High King on other Knight worlds. While the Knightly Houses on Draconis III are Imperial aligned, they tend to have very close relations with the Forge World of Draconis IV. Similar to the Five Hundred Worlds of Ultramar, the Draconis Coalition is made up of over two hundred worlds allied to each other, the Daimyo of each world owing fealty to the Shogun, who in turn rules over the Coalition on the behalf of the Emperor. Draconis III serves as the core world and the capital of the Coalition, and while the largest concentration of Knights gather there, there are minor Knightly Houses scattered across the two hundred worlds in the Coalition, all of whom are distant relatives to one of the three major Knight Houses on Draconis III. The equivalent of a Baron are the Daimyos, Lords and Nobles who pilot the Knights, and the Shogun, as I said, is the Draconian equivalent of the High King.
History of Draconis III and the Great Crusade
Draconis III is a lush agri-world full of greenery, fertile plains, life-giving rivers, streams and oceans, and vibrant forests. The first colonizers immediately set up farming settlements and fishing villages across the four major continents that stretched across an otherwise ocean-populated world, but to protect them from the amphibious Leviathans that rear from the seas, and the monstrous behemoths that assault their relatively small villages, the human colonizers had to depend on the newly constructed Knight suits from the Standard Construct Templates that they had brought along. Along with the Knights we all know, there were also STCs for Titans, particularly Warlord Titans, vast, lumbering god-engines who incinerate the gargantuan monstrosities from the forests with their Sunfury Plasma Annihilators.
The warriors did not care about coexisting with Nature in the way their preceding ruling class did. Focused solely on victory, they trampled upon the grassy plains of Draconis III in their hulking, adamantine armor, followed by columns of armored tanks. Thankfully, they did not have the technical expertise or vision to industrialize their world either, so the ecology of Draconis III remained largely untouched. The Rough Riders used to make up the majority of the soldiers, escorted by marching ashigaru, or foot soldiers, but their ineffectiveness against the colossal armor of Imperial Knights drove them to obsolescence, and the warlike leaders grudgingly acknowledged the futility of sending mere foot soldiers into combat with these giants. What few Standard Template Constructs and manufactorums left were used to construct armored tanks, which allowed even the common soldier to at least wound a towering Knight. A single tank might not be able to put a dent, but an entire squadron of tanks firing in concerted unison could put down a single Knight. This, of course, was limited to the relatively few tanks produced - numbering only in their thousands as opposed to the millions of marching foot soldiers, but it at least reduced the senseless loss of life. The particular military tradition was kept and later served as the foundation for the Planetary Defense Force, and subsequently the Draconian Armored Defenders, which is almost entirely made out of tank companies with armor infantry support. Working in concert, the armor infantry would escort and protect the tanks from anti-tank fire, while the tanks blast huge holes in enemy fortifications and rain fiery death upon massed formations of foes.