One man can start a landslide with the casting of a single pebble

– Attributed to Sebastian Thor

The greatest conflict to engulf humanity was the Great Heresy which began with the Primarch Horus’ virus-bombing of Istvaan III. Although the Horus Heresy saw the destruction of entire worlds and devastation on an unprecedented and unrepeated scale, there are some Inquisitors who see it as one of the moments during which the Imperium and Mankind’s future was forged. From the Horus Heresy came the great military changes that saw the break up of the Space Marine Legions and the division of Navy and Guard, the Emperor’s Ascension and the creation of the Ministorum, and many other events which shaped the Imperium of the present.
It is the goal of the Istvaanians to strengthen humanity through adversity, believing that humanity is at its strongest in times of turmoil and conflict. The periods of greatest upheaval led to the greatest leaps forward in technology, faith, and galactic dominance. Did not the Horus Heresy weed out those Marines disloyal to the Emperor? Did not the Age of Apostasy herald the coming of Sebastian Thor and the reformation of the Ecclesiarchy? Did not the great battles fought by Macharius’ armies shine like a great torch in the dark times of the early 41st millennium! These are the arguments they put forward. The Istvaanians claim that only when it is hardest tested does Mankind show the true strength that lies within it.

To this end, the Istvaanians endeavor to propagate strife and conflict for Mankind’s own greater future. Istvaanians fan the fires of war and hatred so that Mankind will stay vigilant, that its warriors will be skilled and ready, and so none will believe that they have done all they can. They are a bombastic and obvious creed, flaunting their warmongering ways, instilling those nearby with fiery zeal to fight for themselves and the rest of Mankind. They pit faction against faction, world against world so that the survivors will be the stronger for it.
Istvaanians will stir up trouble wherever they go, working on the phobias and prejudices of Imperial commanders, military officers, and the general populace to breed superstition, suspicion, and violence. They work well with destabilization and terrorist cults and foster relationships with violent pro-Imperial cults. Quite often they will build up the power of these secret societies and sects, only to reveal the threat that they pose to the authorities, urging the faithful to bring torch and sword to purge the corruption within. Istvaanians extol the terrors and dangers posed by the forces they face.
Other sects within the Ecclesiarchy do not trust Istvaanians, always questioning their motives and agendas. Opposing Inquisitors argue that, rather than strengthening Mankind, the Istvaanians’ destructive activities weaken and destabilize the Imperium, threatening the bastion on which Mankind’s survival is based. All too often, an Inquisitor may be investigating a heretical cult or individual who is bent on opposing or destroying the Imperium, only to find they are sponsored or led by an Istvaanian who has fostered such rebellion for their own ends. Many such encounters end in physical conflict, as Istvaanians are nothing if not callous and bloodthirsty.