003.M31 - The Luna Wolves are renamed the Sons of Horus - <RECORDS UNAVAILABLE>
003.M31 First War of Xenobia - During the Imperium's feud with the Interex, a highly advanced human society allied with several xenos species such as the Kinebrach, the full panopoly of Warp-based technologies available to their alien vessels among the Kinebrach are unleashed, creating localised Warp Storms, diseases capable of felling even Space Marines in solar minutes and other inexplicable esoterica. The fighting begins when the Interex accuse the Imperials of stealing an artefact, the Kinebrach Anathame, from a local museum of technology on Xenobia. The deed is actually done by the Word Bearers First Chaplain Erebus, who ensures that the artefact eventually finds its way into the hands of the Renegade Planetary Governor Eugen Temba on the moon of the world of Davin.
004.M31 Davin's Moon Campaign and the Fall of Horus - The Warmaster Horus is grievously wounded by a malefic Chaos blade known as the Kinebrach Anathame while fighting Eugen Temba, rebels and the servants of the Plague God Nurgle on the moon of Davin. He is healed within the ancient Serpent Lodge of Davin's moon by its order of priests through the use of Chaos sorcery, but though his life is saved, his soul is forfeit to the Ruinous Powers, who exploit the sliver of jealousy, selfish desire for power and mistrust within Horus' heart to turn him against his father the Emperor. Fearing the influence of the occult on their master, the newly named Sons of Horus fall into schism over the morality of this course of action. From this day forth, Horus makes plans to bring the Imperium to ruin and topple the Emperor from power, replacing him upon the throne of Terra. Though his treachery and malign intent remain hidden, he begins his far-reaching conspiracy to suborn those of his brothers' Legions and the other forces of the Imperium who will aid him and neutralise the threat of those who will not where possible, while preparing in secret to fight a sudden and ruthless civil war. It is later rumoured that it was on Davin that the XVI Legion was first riven and that the Primarch who entered the Serpent Lodge was not the same as the one who left it.
004.M31 Battle of Tarsus - Primarch Fulgrim, leading his 28th Expeditionary Fleet, meets with the renowned Aeldari Farseer Eldrad Ulthran of Craftworld Ulthwé on the Maiden World of Tarsus, where the Farseer attempted to warn Fulgrim of the Warmaster's corruption by Chaos after being wounded by a malefic Chaos blade known as the Kinebrach Anathame, at the hands of the Traitor Eugen Temba upon Davin's Nurgle-corrupted moon. Fulgrim reacted with violent outrage at the Farseer's accusations due to his close kinship with his brother Horus. This outrage was further enhanced by the corrupting influence of Fulgrim's Laer daemonblade, which darkly influenced the Primarch to reject the Aeldari's truth and provoked Fulgrim into launching an unprovoked and furious attack on Eldrad and his retinue alongside his Emperor's Children Captains and his personal Phoenix Guard. In the battle that ensued, the Emperor's Children slew both the revered Aeldari Wraithlord Khiraen Goldhelm and a potent Avatar of Khaine, which forced the Farseer and the other Aeldari troops to sorrowfully withdraw, as they realised that Chaos had already claimed yet another of the "Mon-Keigh's" Primarchs. Yet they succeeded in killing all of Fulgrim's elite personal Phoenix Guard before their departure. Believing the Aeldari had proven themselves a treacherous xenos race that sought to divide and conquer the Imperium by spreading such lies about its leaders, Fulgrim, again under the increasing influence of the daemonblade, ordered the destruction of several other beautiful Aeldari Maiden Worlds using hideous Virus Bombs.
ca. 004.M31 Battle of Deep Orbital DS191 - Following his delegation to the 63rd Expeditionary Fleet, Fulgrim announced to his senior commanders that he would lead a small force to join Ferrus Manus and his Iron Hands at Callinedes IV in the Callinedes System under the pretense of clearing it of an Ork infestation. The Primarch spearheaded the assault during the pacification of Deep Orbital DS191, leading the 1st, 2nd and 3rd Companies. During the assault, Captain Solomon Demeter, commander of the Emperor's Children's 2nd Company, found himself unsupported by the 1st and 3rd Companies as the battle-plan had laid out. Overextended and in real danger of being cut off and destroyed, the 2nd Company was only saved from certain destruction by the unplanned and timely arrival of the 10th and 13th Companies under the commands of Captains Saul Tarvitz and Lucius, two junior officers that Demeter had found himself associating with more and more as he was systematically frozen out by the III Legion's high command. A dark shroud hung over Lord Commander Vespasian after witnessing these dishonourable actions. He felt obliged to act after he observed Solomon Demeter's 2nd Company being intentionally abandoned by both Captain Kaesoron's and Vairosean's companies. It was a decision that would cost the honourable Astartes his life when he was murdered by the Chaos-corrupted Fulgrim himself, who realised that Vespasian would never follow him into betrayal.
004.M31 Vulpa Straits Hrud Migration - Commanding the 51st Expeditionary Fleet, the Warsmith Barabas Dantioch led a large Iron Warriors contingent against a massive xenos incursion known as the Vulpa Straits Hrud Migration. Upon the Fortress World of Gholgis, the Hrud infiltrated and destroyed the Iron Warriors garrison. The intense entropic fields generated by the migratory Hrud swarms caused stone to age and brought flesh to ruin, turning the Astartes warriors into so much dust and bones, rusting their armour and jamming their Bolters as the Iron Warriors' fortress crumbled all about them. Only then did the rachidian beasts creep out of every nook and crevice to attack, stabbing and slicing with their venomous claws. This left Warsmith Dantioch prematurely aged and crippled. Very few survivors made it out alive as Stormbirds extracted the survivors from the remains of Gholgis. Many of the Astartes died from premature aging or were left in aged, superhuman bodies. In the Hrud Migration's aftermath, the worlds of Krak Fiorina, Stratopolae and Gholghis were lost. The Warsmith questioned Primarch Perturabo's prosecution of the Hrud extermination campaign, which left half a Grand Company dead. Dantioch soon found himself relieved from command of the 51st Expeditionary Fleet and permanently garrisoned to the world of Lesser Damantyne for his presumption.
ca. 004.M31 Cleansing of Arigatta - This was an Imperial Compliance action that was carried out jointly by the Sons of Horus, the Ultramarines and the World Eaters Legions against the world of Ariggata. When Imperial envoys arrived bearing word of the Emperor and the Imperial Truth, they were executed in a bloody gesture of independence. Most of the planet was quickly conquered, except for the massive fortress within which most of the leaders of the planet cowered. Eager to be on his way, Horus commanded Angron, Primarch of the World Eaters, to take the citadel and kill only the leaders. Eagerly, Angron led the assault. However, the fortress was heavily defended and the casualties were horrendous, a dozen World Eaters falling for every metre of land taken. Eventually, a ramp of corpses led up to a single breach in the wall, and the Astartes of the World Eaters Legion plunged in and slaughtered all remaining survivors, transforming the fortress into an abattoir of blood. Before Guilliman was able to confront his brother Primarchs regarding the massacre on Ariggata, the Horus Heresy erupted, both Horus and Angron spitting on their oaths of loyalty to the Emperor, and Guilliman's worst fears were confirmed.
004-005.M31 Start of the War in the Webway - Learning of Horus' treachery and agreement with the Chaos Gods to initiate a rebellion against the Emperor through the use of forbidden sorcery, Magnus attempts to warn the Emperor on Terra via psychic projection. In the process, he breaches the wards around the Great Work -- a conduit built by the Emperor and the Mechanicum from the dungeons of the Imperial Palace into an extradimensional xenos network once governed by the Aeldari called the Webway. The Daemons of Chaos flood into the Imperial extension into the Webway from the Warp, forcing the Legio Custodes and Sisters of Silence into a solar-decade-long war to protect the heart of Terra itself.
004-014.M31 The Burning of Prospero - Following his violation of the Edicts of Nikaea in a vain attempt to psychically warn the Emperor of Horus' corruption and planned treachery, Leman Russ is sent at the head of a great censure host comprised of the Space Wolves, the Legio Custodes and the Sisters of Silence to Prospero to bring Magnus the Red back to Terra in chains to face the Emperor's judgement. The Thousand Sons refused to surrender, and their world and works are brought to ruin in a cataclysmic battle which rents the very stuff of space and time. Magnus the Red and the surviving Thousand Sons escape into the Warp where they will soon become the servants of a dark power.
004-007.M31 Purgation of Arkenath - An Ultramarines contingent comprised of five-hundred Legionaries participated in a three-standard-year-long campaign against the Vektate xenos overmind, to free the Human population of Arkenath from psychic slavery.