Alright, time to finally get around to posting my army's fluff, and my first post outside of the introduction area. Hurray!
Part of the story was inspired by The Inquisition War trilogy and Eisenhorn, the first two 40k related books I read. The other part was just the love of certain units on the battlefield and the desire to be able to relate the two in some way.

Here's a basic "origin" type story. I felt torn between being very detailed and very vague here, so the style is probably pretty sloppy. Please excuse it. Any feedback will be greatly appreciated! 8Y

The Beast Within - A radical inquisition sect
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The formation of the Covenant of the Beast took place out on the edges of Imperial space, far removed from Terra and heart of the Imperium. The Trantmoir system was a little known area of space. Within decades of being settled by man the system was immediately lost in a rather abrupt and violent warp storm, cutting off contact from the Imperium. Due to the length of the storm the system was rather rapidly forgotten as anything but a waste of resources and an unsuccessful attempt at colonization.

However, when the warp storm finally subsided, it became apparent that the area could not be ignored. A particular planet, Veriman 1, was giving off a beacon in the warp visible from light years away. The closest response was an inquisitor of the Ordo Malleus, Alirin Kniknet, and his two Interrogators. He moved to investigate with his retinue and a force of men used to put down a small chaos cult from a nearby system. Arrival near the planet revealed that the energy of the warp storm, the mysterious energy source on planet, or possibly both, had caused mutation to spread through much of the planet's population. Alirin, unwilling to declare exterminatus with an unknown, and possibly volatile, energy source of such magnitude on the planet, decided to investigate the situation regardless.
Despite the mutation of much of the population, it appeared Chaos was not rife within their midst. Quite the opposite, one of the main cities of the planet appeared to be besieged by a chaos, a small band of marines and traitor mutants alike, at that very moment, being held off only by a tiny PDF. Without hesitation the Inquisitor's forces moved to the cities defense - the mysterious energy source seemed to be lie at its heart, and whatever the forces of chaos wanted with it, it couldn't be good.

Reports say the small chaos strike force was defeated by the sudden and unexpected attack to the rear, scattering from the assault out into the wilds. Inquisitor Alirin and his acolytes were welcomed by the people as saviors, overjoyed at the envoy from the legendary realm of the God Emperor.

Beyond this point, details of the events become sketchy. Official reports state that a greater force of warships of the great enemy approached the planet, but were shot down by planetary defenses, and that the energy source on the world was contained and destroyed.

However, the more colorful stories say that the Inquisitor was led underneath the city by local guides, to an ancient sprawling complex resembling a massive altar, in which was bound the essence of an imprisoned daemon of incalculable power, bound away for eternity before the Imperium ever set foot on the planet. The stories hold that the energy of the warp storms had been absorbed into the altar, weakening the bindings of the demon's prison and bringing it closer to release into the material world. Some say the Inquisitor used dark warp craft to seal the daemon away and release the energy from the altar. Some far fetched stories even claim he used his own body as a containment vessel for the beast, channeling its essence out of the altar before its physical form could emerge into the world. One thing all the legends agree on is that no planetary defenses banished the invaders, but a massive corona of light shooting from the city that obliterated the orbiting ships.

No one knows the truth, but the latter seems much more plausible. Ever since that time the system has become home to numerous rogue elements, and including a mysterious sect referred only to as "The Covenant of the Beast", as well as many holdings of the now infamous inquisitor...