So, this is what I have so far:

History:

The Vilojaun were the initial rogue human faction that the infantile Arcturan Imperium encountered during its premier military expansion into the median interiors of the known galaxy. 

But just as they are in the latter-day galactic era, the Vilojaun were a bellicose and capricious group of “uncivilized humans”, the astro-tribe possessing a well-earned reputation throughout the Milky Way for frequently substituting acts of rational diplomatic discourse and proper statesmanship with wanton belligerency and turbulent savagery when pressed into dealing with “conquerors”. Their complete disregard for the terms of subjugation espoused by the magnificent Imperial emissary fleet that materialized within the sovereign space of Ni was not only aptly illustrated by an unrelenting storm of blast-fusion cannon fire, but additionally legitimized by an ominous message tight-beamed to a single surviving vessel prior to its hasty warp withdrawal from Ni's dominion:

“[i]Heed the words of our patron, Arcturan filth: the metallic avatars of our divine masters shall litter the ground with your adulterated ichor and rattle the foundations of your precious manors and villas with each stride of their titanic forms. Fourteen empires, each one as aspiring and hopeful as your own, met their fates at the ends of our teeth and talons. Yours will be the fifteenth.[/i]” 

Only three weeks would pass before the fledgling Arcturan Imperium's multitudinous legions and elephantine void fleets clashed mightily with sects of the Vilojaun's barbaric warrior caste. 

The two stellar powers were polar opposites in their respective martial characters: the Vilojaun armed forces sparse in number and chaotic in its organizational composition yet strikingly seasoned and seemingly steeled for confrontation with a significantly larger foe; the Imperium vast and organized yet tactically amateurish and ill-prepared for battle with a far smaller but disproportionately powerful adversary. 

Fashti I began to severely doubt the future prospects of his grandest creation as one imperial system after another fell to the Vilojaun's seemingly unstoppable land armies and formidable void marauder forces.

However, while the Vilojaun's aspect knights and their towering war engines found themselves comparatively uncontested on the surfaces of planets, the space-orientated branch of their armed forces was finding itself increasingly outclassed when pushed to engage heavier Imperial naval assets---most notable of which being Imperial battleships and dreadnoughts: two keystone, war-winning classes of void vessel that the Vilojaun nation could not fabricate in their smaller orbital shipyards in any meaningful number. 

The empire, on the other hand, could afford to produce them at a geometric rate.

This violent confrontation swung increasingly in favor of the Arcturanians with each Vilojaun defeat over a strategically valuable planet or in the treacherous confines of an asteroid belt, its duration spanning over two standard centuries before Radiance Lekaria—the young although illustrious “god chieftess” of this troublesome tribe—would be brought before the negotiation table to outline the parameters of the Vilojaun's conditional surrender and subsequent absorption into the Arcturan Empire. 

In exchange for ensuring the welfare of her people and their right to cultural autonomy, the god chieftess agreed to submit her subjugated nation's entire military arm to imperial authority, agreed to pay a hefty resource tribute to the throne annually, and agreed fully to defer to the will of the royal throne without question nor complaint. 

With new leaders to obey, the old Vilojaun aspect knights and their mechanical idols would be one of many integral causes of the downfall of numerous alien civilizations and the assimilation of scores of independent human societies into the Imperial yolk. 

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Basically, the Vilojaun are an ethnically homogeneous warrior tribe of animal worshipers. They have a thing for creating [URL=http://fc06.deviantart.net/fs70/f/2011/353/4/5/cur_by_zinou-d4jjhou.jpg]gigantic mechanized idols of their patron deities[/URL], distrust most forms of modern technology (especially genetic engineering), and haven't really meshed well with mainstream Imperial culture due to their high distrust of state institutions or [URL=https://en.wikipedia.org/wiki/Kin_selection]anyone who isn't of Vilojaun heritage.[/URL] 

They have proven to be an incredibly fickle people to break spiritually.

Also, despite being techno-barbarians, their comprehension of robotics and metallurgical techniques is exceptionally sophisticated. 

Think of them as a queer amalgamation of 40k's Collegia Titanica, the Aztecs, Gauls, and perhaps the pre-reservation Apache or Comanche Native American peoples. They are probably hated by most imperialized worlds and races on account of their war machines having absolutely outrageous historical death counts attached to them. Old grudges die hard.

Personally, I believe that the key reason behind the downfall of the Vilojaun nation is pretty weak. It would be better if the Imperium actually managed to drum up a worthy mercenary company or something to give the fabled "beast riders" a run for their money. 

They're probably happy as hell that the Imperium is finally dying. God Chieftess Jericha (a direct relative of Lekaria) is probably waiting for the right opportunity to ignite a rebellion of truly enormous scale from Ni, but she won't get far without naval support. Her people were further neutered by the empire when all of their military shipyards were dismantled by Arcturian demolition teams. 

As should be expected, I'm more than willing to tweak the Vilojaun's past so that it better complements the histories concocted by other players.

EDIT: It should be said that a noteworthy integer of Vilojaun would be loyal to the empire. If this nation really speaks to me (and if it can be worked into the setting), expect a lot of Vilo-on-Vilo violence during the course of the Imperium's demise.