"Psychic Awakening: Faith and Fury" story about the Alpha Legion (Apparently this info is only available in the Collector's Edition of the book). The source - reddit via Natfka
-Camera moves again and we are suddenly met with the Alpha Legion in a random part of the galaxy. They are orchestrating the fall of an agriworld. In a matter of hours, the Imperial presence on the planet and within its orbit are devastated by sleeper cells that the Alpha Legions seeded decades ago. The planet is theirs and they begin a massive Chaos ritual of mass sacrifice. Everything is going as planned...except they Alpha Legion couldn't predict the insane calculus of the Iron Hands who through information gathering and logic discovered what the Alpha Legion were intending to do. With no way of warning the Imperials in the sector about the impending doom that's soon going to fall on them all, the Iron Hands sent a force strike force led by an Iron chaplain, one of their finest war leaders. On the way, the Iron Hands are met with a battle sister fleet who also identified the threat on the agriworld but not through math but through a vision.
- Upon landing on the planet, the Sisters of Battle and Iron Hands faced the Alpha Legion cultists. The cultists fought surprisingly well but they could not stop the relentless march of the loyalists. By the time the loyalists reached the main area of the ritual, it was already too late. The Ritual was cast.
-The ritual's outcome wasn't an explosion or a yawning rift. The Alpha Legion went with the subtler route. Psychic energies washed over the planet. The people of the agriworld all were affected by the energies, turning into psykers. 1/3 of the populace died, torn apart by their newfound psychic powers that they could not contain. The 2/3 of the planet's population survived and became psykers one and all.
-Even the Iron Hands were affected by the psychic energies, sending several of them staggering as the psychic signal filled their minds with screams. The sisters of battle on the other hand were immune to the psychic awakening signal. Their minds being too small for doubt (and anything else really) and filled already with faith. The signal could not find any purchase into their minds.
-After a brief skirmish, the Alpha Legion retreated and began an operation of stoking the psychic anguish of the planet with rituals and slaughter. The walls of reality cracked open and the daemons of Chaos poured out. The daemons rushed to consume the psychic populace and these poor unfortunates fled to the Imperial lines. The Iron Hands were prepared to fire indiscriminately on both daemon and civilian. The civilians were channeling the energies of the Warp that are sustaining the daemons' presence, so they had to die. The Sisters of Battle would not allow this so they walked into the sight lights of the Iron Hands and blocked them from firing at the people of the planet, much to the logical chagrin of the Iron Chaplain
-The Alpha Legion took the chance given to them by their daemonic allies to close the noose around the necks of the Imperials. Surrounded by traitor and daemon from all sides, the Iron Hands and Sisters were doomed. But then something miraculous happened. Who would have guessed!?
-The conviction of the Iron Hands and the faith of the Sisters birthed a manifestation of the Emperor. A towering figure with a flaming sword appeared, surrounded by golden eagles. The Emperor Angelic has come!
-The Emperor Angelic swept his sword forward and the eagles came at the daemons burning any they touched with golden fire. The holy energies unleashed haloed the people of the planet turning them into instruments of the Emperor's will. As one the people of the agriworld rose up and walked into the firelines and blades of the daemons. With mere gestures, they sent holy fire and spears of light at the heretics and daemons.
-The Iron Chaplain was speechless at what was happening. This didn't fit any of his logical understanding. Inside his cold soul a long buried emotion sparked and that unnerved him.
-As for the Alpha Legion, their sorcerer leader never saw this kind of psychic manifestation before and was shocked at how his forces were crumbling before it. But he realized that it's ultimately unsustainable. The psychic ritual he started is already straining the fabric of reality. The gathering psychic energies will see the planet to its doom. The Alpha Legion fled the planet with their mission done.
-The holy psykers were named the "Humble Saints" and after the defeat of the Chaos forces on the planets and thousands of the Imperial faith came to witness them. But something had to be done about them. The swell of so much psychic energy did not only threaten the agriworld but nearby worlds as well. The Psychic awakening happening there cannot be allowed to spread.
-The Iron Chaplain argued with that the planet needs to be destroyed for the greater good of the rest of the Imperium but the Sisters of Battle protested. The Emperor Angelic protected the people of the planet and poured his holiness into them. They must be protected at all cost.
-The Iron chaplain could not move the sisters via logic nor any calculations so he decided to get allies better equipped to argue his point. In secrecy the Chaplain contacted the Inquisition which responded in absolute force.
-A fleet of Black Ships swirled around the planet. Imperial stromtroopers descended in droves to help the Iron Hands in herding the population to be processed. Then came in the Sisters of Silence who began to grab portions of the holy psykers for their blackships. Those who would be destined to feed the hunger of the Emperor or sing in his Choir. Others might be turned into sanctioned psykers if they survived the tortures of their captors.
-The whole thing angered the sisters but nothing could be done. This was the order of things when it came to pyskers. Despite the fact these brave souls channel the holy will of the Emperor, they were treated like cattle by the Imperium.
-Then the black warships of the Inquisition began appear in the skies of the world. Their positioning in the orbit of the planet made it clear what was coming next.
-The Battles Sisters contacted the faithful transporters and pilgrims in the system to come evacuate the holy psykers out of the world and send them to the safety of the shrine world of Crenna. A fleet of civilian transports descended on the planet to evac much as possible the holy psychic populace of the planet. The Inquisitorial warfleet could do nothing to stop this, fearing to fire on the relief fleet and cause a civil war with the Imperial Church. Hopeful, yes? But then disaster struck!
-One of the relief ships managed to carry with it 4000 holy psykers guarded by a force of sisters. While it was flying away from the planet, the psykers lost control of their holy powers and the ship burst with golden flames. The ship reactor was breached resulting in a nuclear explosion that everyone saw.
-The Iron Hands intervened and blocked the path of the relief fleet. They threatened that they would destroy every single evacuation ship if the Sisters of Battle don't agree to parley.
-The Sisters agreed and their leader met the Iron Chaplain and members of the inquisition to discuss the fate of the planet. The Sisters of Battle were shown what was happening on the planet. Thousands of holy psykers were losing control of their powers and the psychic conflagration could be seen from space. The fires were spreading from the population centers. The Iron Chaplain spoke bluntly to the Sisters of Battle but without any resentment/malice. The Sisters did well and fought hard. They saved a part of the planet's population but the planet itself and the unfortunate souls on it are doomed. The world needed to be destroyed lest the swelling of psychic energy threaten the whole sector, spreading the madness further beyond.
-Despite the sorrow caused by the decision, the Sisters submitted to the judgement of the Holy Ordos. The command came instantly. The Cyclonic torpedoes went flying into the planet. And so the world of saints was destroyed by those who came to protect it.