The Brotherhood

Isolated from the Imperium's core worlds of Terra and Mars by sheer distance, the Ophelia system was virtually untouched by the Horus Heresy. The rebellion broke fifty-five years after the Emperor's victory against the Garrison, barely two generations after colonization began. The first governor, the man who had been given his orders by the Emperor himself at the mouth of the valley, was still Imperial Commander of Ophelia VII, most of the colonists still living in the initial pre-fabricated domicile-landers. Farmland had been tilled and crops sown and harvested, the first mines sunk into the other planets' surfaces, some factories built and the beginnings of civic infrastructure established, but the system was still in the first phases of colonization. As an unimportant world in the nascent Imperium, Ophelia VII was beneath the notice of the traitors' military command. The barbaric hordes of the World Eaters or the degenerate Emperor's Children might have found a massacre of helpless civilians amusing, but as fate would have it neither of those Legions came near the vulnerable Ophelia system.
The ruling family of Ophelia had followed standard colonization protocols to the letter, developing the system conservatively and with fastidious attention to detail. Cities, factories, farmland and more had spread over Ophelia VII while mines had been sunk into the surface of the other planets in the system. The Emperor's edict to not build or excavate within the mountains had been followed perhaps overzealously; the first governor had lain down a law that no-one even enter the mountain region. Those who had come after him had continued to enforce the edict in this manner. Regardless of this, the system became wealthy and powerful thanks to abundant natural resources, careful management and a long period of peace.
Following his interment in the Golden Throne, cults worshiping the Emperor as divine arose across the Imperium, and Ophelia was no exception. However, whereas on some worlds the cults were tolerated or even welcomed, on Ophelia the government held strongly to the Imperial Truth and punished profession or open worship with fine, imprisonment and even - in the case of cult leaders - torture and death.
Despite this persecution, or perhaps because of it, over the first century of the 31st millennium the cults coalesced into a single belief-system - the Brothers of Ophelia. Like the other Emperor-worshiping cults throughout the galaxy, their theology was based on the Lectitio Divinitatus written (unknown to them) by the Primarch Lorgar. In order to protect members' identities, the Brothers referred to each other only as "Laertes", taken from an ancient Terran text.
But the Brothers' belief in the divinity of the Emperor was not the only mythology on Ophelia VII. Based on scraps of information left behind after the defeat of the Garrison - the redacted campaign records, rumors, speculation and hearsay - legends had arisen about the Emperor's presence on the planet. That the Emperor came down from the mountains alone and met the first governor at the mouth of the valley and placed the mountains under interdict led to speculation of what might be found there. Different groups on the planet believed different things, but the Brothers filtered the scant facts through the lens of their belief in the Emperor's divinity. The fact the governors had been overzealous, forbidding entry to the mountains when the Emperor's decree was only against construction, led them to believe there was some sacred sign in the mountains, something the secular government did not wish to be known because it would reveal the Imperial Truth to be a lie.
In the second century of the millennium, conflict between the Brothers and the government of Ophelia grew and the believers' ranks swelled as more and more people joined them covertly. The most significant of these conversions was a nephew of the Imperial Commander. A far-sighted and well-educated man with great wealth and power, he not only had a strong influence on the cult but had the vision to help guide it.
On Terra, Fatidicus had established the Temple of the Savior Emperor shortly after the climax of the Horus Heresy and now, nearly two centuries later, it had absorbed virtually all the competing cults on the homeworld and most of those in the Segmentum Solar. The nephew knew this was the opportunity the cults of the Emperor had been waiting for; the Temple of the Savior Emperor was more than just one cult among hundreds - its reunification theology was resonating with the people of the Imperium and he knew it was only a matter of time before it absorbed all the other cults in the galaxy. But more than that - as a former-dilettante noble with time and resources to delve deep into such things, he had studied the human condition and the history of faith and knew religion was endemic to humanity. The Temple of the Savior Emperor would not only become the only religion but, sooner or later, the official, state religion. Eventually, the Temple would come to Ophelia and not only atheistic Imperial Truth but any competing cults would be suppressed. It would be better for the Brothers if they joined the Temple willingly - perhaps as equals - and presented Ophelia to the masters of the religion as a jewel in the Emperor's new crown.
The secrecy of the cult made it difficult and he was unable to secure the agreement of all the Brothers' leaders, but he was able to convince them of the value of allying with the Temple and even develop a joint theological statement of understanding. He then declared open membership in the Brothers of Ophelia, publicly taking the name Laertes and professing his faith in the Emperor. He also published the joint theological statement as a manifesto, aligning his theology with that of the Temple of the Savior Emperor. Inspired by his example, many other Brothers also revealed themselves and openly proclaimed the Emperor's divinity.
The secular forces of Ophelia were shocked by the number of Brothers - whole cities and regions were dominated by them. Reacting swiftly, the governor declared his nephew an enemy of the Imperium, the Temple a subversive organization and ordered the military to redouble their persecution of the Brothers, raising the penalty for worshipping the Emperor from a fine or imprisonment to death. However, many members of the military - especially officers - were Brothers and so the persecution was blunted.
Laertes was idealistic and sought to settle the matter without bloodshed - convinced of the divinity of the Emperor, he felt that if the Brothers of Ophelia were free to evangelize, the planet would soon convert. He simply sought an end to the persecution to allow that. But while Laertes was the public face of the Brothers, he was not their leader, and they had other ideas. Their cruel treatment at the hands of the government left them unwilling to be gentle and many of them were angry at what they perceived as Laertes' temerity or even heresy in aligning their theology with that of the Temple of the Savior Emperor. On their orders, a religious uprising took place, with Brothers storming government buildings and military installations.
Laertes appealed for restraint - humans fighting humans was not what the Emperor wanted, and worship was valueless if be coerced. This message resonated with the common Brothers; many of the worst excesses were blunted and the leader's terrorism lost support - among the population at large, of course, but even among their followers. There were open calls for Laertes to be the new governor when his uncle was deposed or stepped down. The Brothers' leaders realized they needed to remove Laertes and plotted to have him assassinated.
The evangelization of Laertes' faction had converted many of the people of Ophelia, and while the governor refused to admit it, it was clear to the secular authorities the Brothers would win the civil war. The only question was; which faction - peaceful evangelization or violent jihad - would dominate. Pushed in that direction by secret Brothers within the cabinet, the secular government determined it was more desirable Laertes' faction be in power and, in a palace coup, deposed the governor and declared Laertes head of state.
Protected by the security forces of the planetary government, the assassination of Laertes failed. But the attempts forced his hand; the jihadist faction's motives were laid bare. It was clear they did not simply desire to be permitted to worship openly, or even to have their religion dominate the planet - they sought vengeance and violence against those who had wronged them, and would kill anyone who got in their way. With great regret, he ordered the destruction of the jihadist faction and - armed with not only the full strength of the Imperial military, but also Laertes' knowledge of the Brothers' internal structure and practices - they did just that.
Some of the Brothers never forgave Laertes for that treachery and when he assumed the leadership of the Brothers, formally merging them into the Temple of the Savior Emperor, that was the final straw. A schism occurred, with a faction of the Brothers unwilling to give up their independence and making mountains out of theological molehills splitting off and setting themselves up against the Temple of the Savior Emperor.
But this was a minor wrinkle in the smooth transition of Ophelia from a conservative, atheistic system devoted to the Imperial Truth to one devoted to the worship of the Emperor. Laertes was named Cardinal by the Ecclesiarch of the Temple of the Savior Emperor and Ophelia VII became the first of the Cardinal Worlds, with special privileges and entitlements. Laertes had achieved his goal; as the power and influence of the Temple of the Savior Emperor grew, so would Ophelia's.