The Shadow of the Emperor was a Gloriana-class Battleship that served as the flagship of Corvus Corax, the Primarch of the Raven Guard Legion during the Great Crusade in the late 30th and early 31st Millennia. This flagship led the flotilla of the Raven Guard warships which comprised the XIXth Legion's 27th Expeditionary Fleet.
A notable feature of the Shadow of the Emperor, present on all Raven Guard vessels, was the use of a special device known as a Reflex Shield, a modified version of the Void Shields that were utilised to protect most Imperial warships and void-based installations. This device was an innovation first developed by the Mechanicum on the Forge World of Kiavahr, the world orbited by the airless moon of Deliverance which was the homeworld of the Raven Guard Legion.
A Void Shield worked by using the power of the Warp itself to displace incoming projectiles and directed energy attacks through gravitic manipulation. Reflex Shield technology altered the modulation of the Warp-Drive cores that powered the Shadow of the Emperor 's Void Shields, calibrating them to a much higher tolerance and also directing them inwards, so that all of the matter and energy emitted by the great warship was redirected instead. All forms of radiation emitted by the Raven Guard's starships could be displaced, rendering them undetectable to scanning equipment.
The advantages of the Reflex Shield technology fitted well with Corax's ethos of war, allowing Raven Guard starships to approach their targets unseen, and strike swiftly and decisively before withdrawing. The low energy requirement for the use of the technology meant that such stealth could be maintained almost indefinitely.
There was, however, a serious downside to the use of Reflex Shields. They significantly reduced a ship's sensor capabilities, essentially making them half-blind. Also, by employing its Void Shield generators for the projection of Reflex Shields, a Raven Guard vessel had no defence against physical attack and it took time to switch the generators from one state to the other, leaving a warship vulnerable for several solar minutes with neither its cloaking field nor its energy barriers fully operational.
Another problem with Reflex Shield technology, one that Corax had unsuccessfully laboured to overcome for many years, was the relatively low amount of energy emissions it could cloak. The vessel's Plasma Reactors could only be run at half power or they generated too much energy to be fully displaced by the Reflex Shields. Thus, when operating under the cloak provided by the technology, a Raven Guard starship had to reduce its top speed and make do with decreased sensor capabilities.
To the Augurs and scanning arrays of an orbital base or starships throughout a star system, the Shadow of the Emperor or any other starship activating Reflex Shield technology would seem to melt away into the stars. To the naked eye it would appear to shimmer, as the Reflex Shields engaged and reflected all visible light from the ship's surfaces, until eventually all such energy was dampened and the vessel was rendered effectively invisible to sensors and the naked eye alike.
When making for a planet, the Shadow of the Emperor would not take a direct route, it being a doctrine of the XIXth Legion to always approach a target indirectly. Instead it would take a circuitous, zigzagging path, using a timing and distance formula devised by the Primarch himself to maximise the damping effect of the Reflex Shields and throw off any pursuer or sensor that might somehow detect them. Corax did not believe in taking chances when it came to moving freely and unseen.
In response to the treachery of the Warmaster Horus and his betrayal of the Loyalist Astartes in the Sons of Horus, Emperor's Children, World Eaters and Death Guard Legions at Istvaan III, the Primarch of the Imperial Fists Legion, Rogal Dorn, on the direction of the Emperor who had learned of Horus' actions from the Loyalist survivors aboard the Eisenstein, ordered 7 Loyalist Space Marine Legions to Horus' base on the world of Istvaan V to challenge the Traitors. They would attack in two waves and fall under the supreme command of the Iron Hands' Primarch Ferrus Manus.
The Legions comprising the first wave were the Iron Hands, Salamanders, and Raven Guard. The Legions comprising the second wave were the Alpha Legion, Night Lords, Iron Warriors, and a large contingent of Word Bearers that their Primarch Lorgar had stationed in the star system. Unknown to Dorn and Ferrus Manus, the Night Lords, Alpha Legion, Iron Warriors and Word Bearers had all turned from their service to the Emperor and pledged their loyalty to Horus, and been instructed to keep their new allegiance to Chaos a secret.
The Iron Hands, Salamanders and Raven Guard were deployed in the first wave of the assault on the planet while the Shadow of the Emperor and the rest of the flotilla of Loyalist vessels provided overwatch of the planetary assault, standing by in case they needed to provide assistance to the attacking ground forces. After they secured the drop site, they were to have been followed by the arrival of the other four Legions.
The first wave won the drop site at a heavy cost. Horus ordered his front line troops to fall back, tempting Ferrus Manus to overstretch his already thin lines. Against the advice of Corax and Vulkan, Manus led his Veterans against the fleeing Traitor Marines unsupported. Manus then brought his brother Fulgrim to combat.
As the two Primarchs drew their weapons, the Raven Guard and Salamanders fell back to regroup and allow the second wave's Legions to advance and earn glory. However, as they returned they were mowed down by the four Traitor Legions that had landed to supposedly support them, thus revealing their new allegiance to Chaos. Horus then pressed his own attack and sandwiched the Loyalists between the two Traitor forces, killing most of them.
When the Traitors opened fire on the first-wave of Loyalist Astartes, the Shadow of the Emperor received their stand-to and distress broadcasts. It was cut off within a matter of solar minutes, jammed by the Traitor warships in orbit. It was too soon for the Reflex Shields to be raised, and against such numbers that were arrayed against them, that would have been the only defence.
The Terminus Est, under the command of First Captain Calas Typhon, was one of the few capital ships of the Death Guard Legion. When the Traitors revealed their treachery, the Terminus Est suddenly engaged and treacherously destroyed the unprepared Shadow of the Emperor. It is argued by many Imperial scholars that this engagement was the earliest recorded conflict between Battleships that were specialised to carry Attack Craft.
The engagement was swift and brutal, depriving the embattled Loyalist forces on Istvaan V of any air support in the massacre that followed. The loss of the Shadow of the Emperor is remembered to this day as one of the greatest losses of the Raven Guard in the Horus Heresy and the Chapter maintains a long-standing grudge against the Death Guard as a result.
- Deliverance Lost (Novel) by Gav Thorpe, pp. 22-23, 27