History of the Galaxy

The Drop Site Massacre of Isstvan V, also called the Battle of Isstvan V, was one of the major turning points that occurred early in the great galactic civil war that engulfed the Imperium of Man in the early 31st Millennium known in later times as the Horus Heresy.
After the news of the Isstvan III Atrocity was brought to the Regent of Terra Malcador the Sigillite and the Primarch Rogal Dorn, the Praetorian of Terra, by the Loyalists aboard the Death Guard frigate Eisenstein, the Emperor of Mankind entrusted Dorn with the creation of a plan to deal with the rebellion of the Warmaster Horus and his allies.
Under the Emperor's authority, in ca. 006.M31 Dorn ordered the combined forces of seven Space Marine Legions to assault the positions of Horus and his Traitor Legions in the Isstvan System.

In 211.M31 Russ, at a great feast with his Legion in attendance, stood and said: "In the end I will return. For the final battle. For the Wolf Time." He then left the Fang and embarked into the Eye of Terror accompanied by his personal retinue bar one — Bjorn the Fell-Handed, who remained on Fenris in the role of Great Wolf. The tale of his disappearance is retold every thousand years by Bjorn, now the oldest dreadnought alive. Every once in a while, the Space Wolves embark on what is called the Great Hunt to search the galaxy for their missing leader. Though they have searched far and wide, they still haven't found any sign of their Primarch. Many within the Space Wolves believe that Russ has sought out the Tree of Life to restore the Emperor to health. Apparently the Wolf Primarch's arch-nemesis Magnus knows the Wolf King's whereabouts, but keeps it secret.

According to some sources, betrayed by agents of his rivals among Lord Militant Angevin's generals, Drusus was attacked in 367.M39 by a deadly assassin whilst rallying his depleted forces on the world of Maccabeus Quintus and was seemingly slain, before rising again -- an event many saw as a true miracle and a clear mark of the Emperor's favour. The Drusine sect of the Imperial Cult began to flourish in his shadow, already revering him as a living Saint, while shadowy agencies, some say belonging to the Ordo Malleus of the Inquisition, also brought new aid to his forces in the persecution of the Yu'vath and their human allies (which included Traitors within the Imperials' own ranks).

Lord Militant Golgenna Angevin died at his palace on the world of Quaddis in 372.M39. Though the official cause of death was listed as natural causes, rumours lay his decline in health on advanced old age and a surfeit of fine living. Darker stories persisted of the Officio Assassinorum's hand in matters as punishment for his later failings in the prosecution of the Angevin Crusade. Drusus was named Lord Militant by wide acclaim in Angevin's stead (with the political support of both the Departmento Munitorum and the Inquisition) and as soon as a state period of mourning for the late Angevin was over, Drusus immediately set to re-forging the region's military forces for a final counterattack into the regions of the Adrantis Nebula and the remaining strongholds of the Yu'vath Hell Worlds.

The first and greatest Lord Sector Calixis dies and is succeeded by Marshal Corin Shultus in 417.M39, his former aide-de-camp and distant kinsmen to the late Golgenna Angevin. The final resting pace of Drusus' mortal remains is kept a secret, although rumours circulate that he was taken back to Maccabeus Quintus to be interred at the site of his first "death." Mass lamentation and political unrest accompany the news of his passing and the entire sector undergoes a seven-year cycle of mourning.

The exact origin of the Tyranids themselves is unclear, save the fact that they are not of The Galaxy and have only recently arrived here after traveling countless millennia in the intergalactic void. It is unknown which galaxy they originated from, or for how long the Tyranid race has been on its genocidal rampage, but it is believed that it is the Astronomican that is drawing the Tyranid Hive Fleet to threaten the galaxy. The Tyranids were first attracted to the Milky Way Galaxy when the xenos communication device known as the Pharos was overloaded in the Battle of Sotha during the Horus Heresy. Indeed their very name is but a title given to them by the Imperium, named after the planet where they were first encountered (Tyran). It is possible that they have been preying on other galaxies since time immemorial. According to another source, they have consumed one thousand galaxies and are responsible for the annihilation of millions of intelligent species.

Terra is assaulted by the forces of Khorn in it's first major invasion in 10,000 years.

A hyper conservative faction of former members of the Senatorum Imperialis attempt to launch a Coup to overthrow Guilliman while he is away waging the Indomidus Crusade.
Needless to say they all die... horribly... those who remained alive were quick to endorse his new title as Imperial Regent.

A massive relief force descends upon the world of Iax. Guilliman defeats his brother on the surface of the world with the aid of The Emperor's direct intervention in their conflict. Guilliman is able to permanently burn down portions of Nurgle's garden, and Mortarion is punished by his master for his abject failure.