First Contact With An Angel
Early 31st Millennium - Horus was wounded by the Anathame blade and entered the Temple of the Serpent Lodge and was shown a vision of the future by the Traitor Space Marine Chaplin Erebus of a horrible future and another vision by his brother Magnus the Red of Horus's duty and place in the universe. After each vision The two confronted him to hear his decision, Magnus pleaded with Horus to remember their father and all their dreams for the future. Erebus tried to manipulate the injured Primarch. But in the end, Horus decided to choose his brother Magnus and trust in him. Thus after that Horus accepted his death and in his dying breaths sent a transmission saying who he wished to succeed him as Warmaster. His brother Sanguinius. Horus perished upon the alter never giving in to the temptations of the Dark Gods, much to the rage of the Traitor Erebus and the discontent of those Dark Gods.
Early 31st Millennium - Two Terran months after the death of Horus. Lorgar had to accelerate his corruption of the other Primarchs he successfully turned Mortarion and Angron to his cause. But in the attempt to corrupt Fulgrim, the Emperor's Children Apothecary Fabius Bile arrived and helped turn the Primarch from the ensnarement of the Laer Blade and saved him. This ultimately leads to Lorgar being outed as a traitor and worshipper of the Dark Gods by his brother Fulgrim.
Early 31st Millennium - Three Terran months after the death of Horus. Magnus the Red was pardoned for his use of Sorcery and the dark arts. That same month the Nikea Edict was reversed with the help of the Primarch Lion El'jonson, Jaghatai Khan, and Corvus Corax. That same month in an act to secure Perturabo's loyalty the Emperor, with the Primarch Sanguinius's assistance, decided to name Perturabo the Lord Architect of the Imperium. He would work in tandem with the also newly named Lord of Logistics Roboute Gulliman. The two would develop over the years a deep-rooted respect for one another because of this. Around the same time Magnus the Red, to also secure his loyalty, would be Named Lord of the Webway, where he would work to complete the Webway Project with his Father.
Early 31st Millennium - Four Terran months after the Death of Horus. What is now called the 'Awakening' occurs. The Primarchs and the higher echelons of the Imperial Military are made aware of the Dark Gods and the existence of daemons and the supernatural forces of the Immaterium.
Early 31st Millennium - Six Terran months since the death of Horus. The first battles of what has been dubbed Lorgar's Rebellion. The primary fight between the Blood Angels, Custodians, Imperial Fist, and Space Wolves versus the Word Bearers, Death Guard traitor elements, and the World Eaters. The Battle ended with the death of Erebus to the Primarch Rogal Dorn and Angron was severely injured by Sanguinius in a duel between the two, after the death of the Chaplin the forces of Chaos retreat to unknown regions on the outskirts of imperial space near the Halo Stars.
Early 31st Millennium - Eight Terran Months after the death of Horus. The Fabricator General of Mars Kelbor-Hal sides with Lorgar in the Rebellion, taking the chance to betray the Emperor. This was eventually called the Hal Schism in which only a small part of the Mechanicum sided with Kelbor-Hal. The rest mostly rallied around Zagreus Kane and Koriel Zeth who would join their forces to fight the traitors of the self-titled Dark Mechanicum. Hal ultimately being slain by the Primarch Leman Russ, The Emperor's Executioner, who came to the aid of Koriel and Kane and at the order of the Emperor. Hal's followers fled after this and departed to the Eye Of Terror not to be seen until later on in the Rebellion.
Early 31st Millennium - One Terran year after the death of Horus. Sanguinius is named Lord Commander upon his request to not take the mantle of Warmaster. No forces of Chaos were found aside from small recruiting parties, all are killed that are found, most being raving lunatics. To combat the attempted recruitment of Imperial Citizens the Emperor, with the assistance of his friend Malcador the Sigillite, founded an organization to fight the servants of The Dark Gods and the predations of the Xeno, this force would be dubbed The Inquisition. Also around this time, Koriel Zeth is named as the new Fabricator General, with the help of an endorsement from the Emperor and the help offered by her friend Zagreus Zane. After this Zeth and a Terran girl by the name of Dalia Cythera embarked on a quest to find the Noctis Labyrinth.
Early 31st Millennium - 2 Terran years after the death of Horus. Angron's corpse found on the world of Alena. After an autopsy was conducted the cause of death is attributed to the Butchers Nails ultimately ending the Red Angel's life. That same year the newly founded Inquisition finds trace evidence of a Xenos cult trying to recruit Imperial Citizens with the promise of money and other bribes. The Emperor upon hearing this sends forth his best sons for hunting down the Xenos cult from the shadows they spawned from, The Primarchs of the Alpha legion. Koriel Zeth and Dalia found the Labyrinth where they were met by The Guardian of the Dragon, Semyon, who would Ultimately name Dalia and a Skitarii in their company as the new Guardians of the Dragon. Koriel would return to the surface and only ever tell very few of these events, she would then take up the full responsibility as Fabricator General and bury any news of the Labyrinth. She would begin to make radical reforms within the Mechanicus.
Early 31st Millennium - 3 Terran years after the death of Horus. Traitor forces were reported to be amassing on the outskirts of the Segmentum Obscurus near the Halo Stars. The Traitors cut a bloody swath in the Segmentum only being stopped by the combined efforts of the Ultramarines, Dark Angels, Salamanders, and White Scars. The traitors were discovered to be attempting to amass enough death to transform Lorgar into a Deamon Primarch, ultimately failing the attempt and losing his other brother Mortarion in the process to their brother Jaghatai Khan. this would later be called the "Last Gambit" by loyalist forces. Many take note that the World Eaters were absent for the assault. Lorgar's remaining forces retreat back to the Halo Stars and disappear not long after making entry into the area of space.
Early 31st Millennium - 4 Terran years after the death of Horus. No traitor forces had been seen since the "Last Gambit" and the Imperium begins to move on somewhat, but never the Primarchs always lurking and looking for the remaining Traitor forces. New Edicts begin being placed into effect. The most popular and widely received is the New Hope Edict or the "Rebuilding Edict" as it is dubbed by the general populous. The edict entailed the rebuilding of wartorn worlds and the "Taming" of Death Worlds, reconfiguration of Death Worlds, or relocation of Death World inhabitants. The first world affected by this change was the world of Baal home of the Lord Commander. The Edict was ultimately put under the Jurisdiction of the two Primarchs Rogal Dorn and Perturabo in an effort to help the two get along. The two would ultimately work together and also were able to put aside thier animosity towards each other. This would ultimately culminate in the two rebuilding the Imperial Palace, together. The new Palace would be completed only after 2 years thanks to the brother's combined efforts. The two would develop something of a bond through this endeavor, thus at least putting a pause upon their feud.
Early 31st Millennium - 5 Terran years after the death of Horus. The Traitors strike were the Imperium expected it least. Traitor forces stormed the Uhulis Sector, primarily ravaging the world of Krieg. 2 months after the initial siege the world is saved by imperial reinforcements in the way of the Blood Angels and Iron Warriors. The bulk of the World Eaters were defeated and killed off for refusing to back down and run even in the face of sheer defeat. The World Eaters only ran at the Death Of Kharn the Destroyer to the Primarch Perturabo.
Early 31st Millennium - 6 Terran years after the death of Horus. The World Eaters lead by Crull strike again this time attempting to get under the Imperiums nerves by attacking the world of Baal trying to kill off the Blood Angels homeworld. The defenders push the heavily depleted legion off the world and keep them primarily in the void for the duration of the 2 week-long siege. The Blood Angels came with a vengeance. With Raldron the Legion Master ultimately slaying Crull on the bridge of the World Eater's flagship, Conqueror the ship crashing into the dead side of the world of Baal. the wreck was later destroyed by the Mechanicus never to be used again because of its Chaotic Taint. After this crushing defeat, the Traitors made a hasty retreat to the Eye of Terror. Not to be seen until the culmination of Lorgar's Rebellion.
Early 31st Millennium - 10 years after the death of Horus. The Mechanicum under the leadership of Koriel Zeth begins looking into Xenos artifacts, with the more puritan Tech-Priests being mysteriously silenced and disappearing. Koriel eventually is able to work and develop a scientific development branch within the Mechanicus dedicated to the discovery of long lost sciences and new innovations.
Early 31st Millennium - 14 Terran years after the death of Horus. The Imperium had mostly moved on only the Primarchs and a few Generals watched to see the Traitors rear their ugly heads and were prepared, most people simply thought the Traitors had died in the void or ran away from the sheer odds stacked against them. Sadly they were wrong the ultimate play was to be called. The Traitors had secretly been corrupting the Night Lords Legion to Chaos and at this moment they turned on the Imperium. Around this same time, Konrad Curze and the remaining loyalist elements disappeared, most people believing him assassinated by his own sons. The Chaos forces put all their might into this final attack killing billions of innocents in the ensuing battles raging across the Obscurus Segmentum. It was at this moment Lorgar "Ascended", he became a Daemon Primarch from the death of all those souls as well as the Champion of the Dark Gods. The loyalist that answered the call first were the Space Wolves, Dark Angels, Thousand Sons, and the Raven Guard. In the aftermath of Lorgar's and his "High Priests" "Ascension" the war became a stalemate for a time for both sides, neither able to get a decisive strike on the other. The tide would turn when the Emperor, Sanguinius, Perturabo's and Rogal Dorn's forces arrived landing a mighty blow to the Traitors. The Emperor permanently slaying several of Lorgar's High Priests, eventually Dueling the Primarch himself. Lorgar was almost felled by the Emperor's blade but was saved by the last of his High Priests, Kor Phaeron, who took the blow for him. After the death of his last Priest, Lorgar ran with the remaining traitor forces he had left and ran all the way to the Eye itself for safety with his dark gods.
Early 31st Millennium - 17 Terran years after the death of Horus. The last of the Xenos cult calling itself the Kabal were hunted down secretly by the Emperor's two sons Omegon and Alpharius. The same year the Golden Throne was completed. Thus making it far harder for the forces of Chaos to summon their daemonic allies. Through the completion of the Golden Throne, the Webway of Mankind is finished, ahead of schedule thanks to the assistance of Magnus the Red, and its modes of transport begin to be made for mass production primarily being sent to the Legions first then to the Imperial Navy.
Early 31st Millennium - 18 Terran years after the death of Horus. The "Last Offensive" happens the Traitors attempted one last sally forth from their hideouts to try and kill the Emperor and his Lord Commander in a last-ditch effort to stop them, both attempts ending in utter failure. The Traitors were still too broken in number and strength so they were easily crushed without reliable aid from Daemons. Lorgar himself being Banished back to the warp by Sanguinius and his son Azkaellion.
This will be the timeline for now but it will be updated as I learn of more that should be added.
10/19/2019: I added the Inquisitions founding and its first exploits into stopping the predations of the Xenos.
11/202019: I have added more with the Mechanicus and the Primarchs including Perturabo, Roboute Gulliman, Magnus, Leman Russ, and Rogal Dorn