The Evocati were a specialised formation of the Ultramarines Legion during the Great Crusade and opening days of the Horus Heresy in the late 30th and early 31st Millennia. They were comprised of several thousand Astartes drawn from a dozen Chapters, awarded the highest honour of all: overseeing the operation of the XIII Legion's War World Armatura and the training of new recruits, commanding an Imperial fleet to rival any other. In the wake of the Word Bearers' treachery at the Battle of Calth, the XVII Legion, led by their Primarch Lorgar, linked up with the Primarch Angron and his World Eaters Legion to launch a Shadow Crusade against the ancient Realm of Ultramar's Five Hundred Worlds in an attempt to spread the massive Warp Storm known as the Ruinstorm that had been conjured by the Word Bearers at Calth across the Eastern Fringe. This would split the galaxy in half and deny needed reinforcements to the Loyalists as Horus drove on Terra in an attempt to overthrow the Emperor of Mankind. Armatura would be the first world to fall in the wake of this brutal joint campaign. It was the Evocati who stood in the way of the Traitors' assault upon Armatura. It is unknown if some form of the Evocati still exist in the late 41st Millennium.
During the opening days of the Horus Heresy, Lorgar had ordered his two most trusted advisors, First Chaplain Erebus and the Dark Apostle Kor Phaeron, to unleash their wrath against the Realm of Ultramar. This was done in retaliation for the humiliation the XVII Legion had been forced to endure by being forced to kneel in disgrace before the Emperor and Roboute Guilliman and his Ultramarines on the world of Khur by the XIII Legion at the Emperor's orders during the Great Crusade. The Word Bearers proceeded to achieve a monumental victory at the Battle of Calth which ensued. The Ultramarines Legion was badly crippled and no longer presented a viable threat to Horus' plan to drive on Terra. Erebus had managed to complete his blasphemous ritual on Calth's surface, which summoned the beginnings of the sorcerous Ruinstorm to the galaxy's Eastern Fringe -- a monstrous Warp Storm larger and more destructive than anything space-faring humanity had witnessed since the days of the Age of Strife.
Simultaneously with the Word Bearers' assault on Calth, Lorgar and the more reliable Word Bearers under his command launched a second offensive, a joint Shadow Crusade with his brother Angron's World Eaters Legion into the rest of the Realm of Ultramar, laying waste to the Five Hundred Worlds with reckless abandon, slaughtering twenty-six worlds in rapid succession. This was to ensure the success of the sorcerous Ruinstorm, which would ultimately split the void asunder, dividing the galaxy in two and rendering vast tracts of the Imperium impassable for centuries, effectively cutting Ultramar off from the rest of the Imperium. This prodigious Warp Storm would deny needed reinforcements to the Loyalists as Horus drove on Terra in an attempt to overthrow the Emperor of Mankind. Nothing from Terra would get in and nothing would get out. Not even an astropathic whisper would be able to pierce this storm of Warp energy bleeding into realspace.
Armatura turned in the heart of Roboute Guilliman's perfect empire. Neither the crown jewel that Macragge claimed to be, nor the future capital Calth had threatened to become, Armatura matched both in importance, and vastly eclipsed them both in population. If Ultramar was reduced to crude metaphor, Macragge beat as the heart of the astral kingdom, while Calth served as its soul -- a sign of a bright future, later consigned to fire. Armatura was a War World, feeding the other planets the way bone marrow feeds blood into the body. It fed the XIII Legion with recruits; it fed the void with damaged warships reborn from its docks; it fed the Imperium with hope that the largest Legion would forever be the largest, and even if the XIII was reduced to a single warrior, as long as Armatura turned in the night, the Legion would live on. Its close-orbit played home to immense shipyards, populated by thousands upon thousands of workers, Servitors, archimechs, Enginseers, serfs, thralls and technographers. It took an army of souls to breathe life back into the great warships of the Imperium, and in the orbital shipyards at Armatura several million of them did their finest work. Orbital bastions of linked gantries and docking maws drifted above the placid world, crawling with insectile shuttles, lifters, loaders and tugs. Imperial warships limped here, scarred from the Great Crusade, and left only solar months later in resurrected perfection. Above and beyond the shipyard was the first concentric ring of void defences. Here, weaponised satellites and fire platforms bristled with turrets, alongside independent landing decks for fighter craft in lockdown.
Beyond those, the true defences began -- castles in the sky: great fortress-stations with their own racks of fighters and entire battlements given over to plasma batteries, laser broadsides and shipkilling Lance arrays. In highest orbit, the outer sphere of satellites was a three-dimensional spread of solar panels, clockwork engines and slaved Servitor brains all connected to vast long-range weapons arrays. Amidst that outermost sphere waited the Evocati fleet. While the rest of the Legion mustered at Calth, the XIII Legion's War World could never be left undefended. The Evocati was comprised of several thousand Ultramarines drawn from a dozen Chapters, awarded the highest honour of all: overseeing the operations of Armatura and the training of new recruits for their largest of all the Space Marine Legions, commanding an Imperial fleet to rival any other. The vessels moved in a perfection of militaristic motion above Armatura that even their enemies found beautiful to behold.
On the planet's surface the Evocati were protected by the formidable forces of the Armaturan Academy Guard, the XIII Legion's barracks-cities, for its Initiates and Evocati overlords as well as the Titan Legion of the Legio Lysanda. Armatura was also protected by literally a billion human soldiers, not even counting the Titans or Mechanicum Skitarii forces. This accounting did not even cosider the tank battalions stationed on the planet as well as the thousands of Ultramarines Evocati. These geo-conflict analytics came from Ultramar's own census archives. Even though it was a handful of years out of date, by the beginning of the conflict, the Traitors still faced a billion soldiers at Armatura. Even if a tenth of them were teenage youths in the earliest stages of Astartes gene-implantation, the Traitors' assault would be far from a bloodless triumph. Led by their formidable War-Regent, Captain Orfeo, the Evocati made it difficult for the invading forces at every turn. As the World Eaters made their way inwards towards the barracks-cities, the Evocati made them pay in blood for every inch of ground gained. As they made a tactical withdrawal towards their carefully prepared defences, they rigged the city's buildings to explode, raining down tonnes of debris and rubble to bar their way.
It appeared that Lorgar's plans to assault Armatura were for naught, for to attack the War World the Word Bearers would need a vessel to rival anything humanity had ever wrought. The Word Bearers had possessed such a vessel once -- the Furious Abyss. But it had been destroyed only solar days before the start of the Traitors' assault, close to the same moment Kor Phaeron's expeditionary force had struck Calth. Its corpse was probably still a shadow in the skies of Macragge, a monument to the Word Bearers' failure. But Lorgar had been underestimated, for he had planned for just such an eventuality. He had been planning the events that led up to the Horus Heresy for nearly half a Terran century. Lorgar's foresight became apparent when a vast trident of dark metal emerged from Warpspace near Armatura, a great warship whose shape was immediately familiar to the Word Bearers fleet approaching Armatura.
The starship that emerged into reality was a reflection of the slain colossus once called the Furious Abyss. A veritable city of monasteries and cathedrals rose from its back with the reverence of clawed hands sculpted to clutch at the stars. Where most Imperial Battleships were spears of crenellated intent and iron-ridged might, this was a fortress in space, borne on the back of a great trident. The central tine served as the vessel's core: dense at the stern, encrusted with massive engines and tapering towards the prow, where it formed a pointed ram the size of lesser vessels. The trident's adjacent tines formed smaller blade-wings, each one barnacled with laser broadsides and Macrocannon batteries. If one were to clad the concept of spite in iron and set it sailing amongst the stars, it might approach the image of what burst back into the universe in that moment. It was, in every way, the Furious Abyss reborn. This mighty vessel was the Blessed Lady.
This colossus was named for the Word Bearers' former Confessor, Cyrene Valantion, the Confessor of the Word, and the lone survivor of the destruction of the Perfect City of Monarchia at the hands of the Ultramarines on the world of Khur decades before. The Blessed Lady easily eclipsed the Gloriana-class Battleships used as flagships by most of the other Space Marine Legions. But Lorgar's final secret was yet to be revealed. He had not had only two of these mighty vessels built in the Mechanicum shipyards orbiting Jupiter. As a second Warp-slice ripped across the stars near Armatura, another colossus was revealed. This was the Blessed-Lady's twin sister-ship, the Trisagion. The pair of Traitor Dreadnoughts rivaled even the Imperial Fists' Primarch Rogal Dorn's precious Phalanx in size and firepower. Lorgar had secretly had three of these mighty vessels built for the service of the Word Bearers.
The Blessed Lady and her twin sister, the Trisagion, made a mockery of Armatura's orbital arrays, dismantling one of the best-defended worlds in the Imperium with barrage after barrage from their howling, flashing weapon decks. The ships’ size and scale rendered all countermeasures obsolete. For the first hour, nothing could punch through their Void Shields. Nothing even managed to scrape their skin. It took the combined firepower of a battle-station, two orbital defence platforms and a suicidal ramming from an Imperial warship to finally penetrate the Blessed Lady's shields. She sailed on, oblivious to the thousands dying within one of the flaming monasteries on her back, for their agonies made no difference at all to a crew composed of half a million mortal men and women, all singing the praises of Lorgar and the Chaos Gods. The Word Bearers' mighty Battleships made a mockery of the Ultramarines' defences and helped crush the Evocati fleet and win the day at Armatura for the Traitors.
On the ground, the World Eaters, fighting alongside the Traitor Titans of the Legio Audax, crushed all that stood in their way in a brutal and bloody campaign against the formidable Evocati rearguard. Bolstered by the berserker Primarch Angron, who led his forces from the front, in the wake of the XII Legion's bloodlust and berserker rage the defences of the Ultramarines fell and the Evocati were slaughtered. The XVII Legion mutilated any severely wounded Ultramarines, as others were dragged away towards their scarlet tanks and gunships displaying the new colour scheme soon named "Traitor's Red" by their foes, to face an even grimmer fate. The Warp around this world boiled from the amount of suffering that had taken place across its cities, enough to draw the eyes of the Chaos Gods themselves.
- Betrayer (Novel) by Aaron Dembski-Bowden