Codex: Chaos Space Marines (2012), p22-25 — The Long War

The Long War has raged for ten thousand years. The bitterness and spite of the Chaos Space Marines has spread throughout the galaxy, spilling from the Warp and bleeding back again for time immemorial. Though the Inquisition takes great pains to quell reports of Chaos incursions in order to spare the common populace from unconventional thoughts, the legends and records persist across the galaxy, handed down through the generations by the foolish and the brave.

31st Millennium

[...]

M31 Horus Heresy

The galaxy is torn by bitter rebellion. Horus and his Traitor Primarchs lay waste to the Legiones Astartes and all but overthrow the Emperor of Mankind. The forces of Chaos are eventually hurled back from Terra and most are driven into the Eye of Terror.

M31 The Long War Begins

Thirsty for revenge, the Chaos Space Marines strike out against the Imperium from the depths of the Eye of Terror. The Adepts of the Administratum are horrified to find that the number of their invasions consistently increases with every passing decade.

[...]

32nd Millennium

c.400.M32 Perturabo's Plague

The Daemon Primarch Perturabo perverts the eight rituals of possession, turning them against his enemies. Invoking Nurgle, Perturabo imbues his curse with extreme contagion and releases it into the mechanical systems of Toil, a vassal forge world. The raw Chaos spreads through the machines, and the hidden manufactorums begin to change. On the eighth day, giant cables burst from the earth, daemonic machines hunt the living, and many-legged cathedrals of industry prowl the wastes. The planet is ultimately scoured of all native flesh.

[...]

36th Millennium

437.M36 The Green Death

The infected Ork warbands invading the Ecclesiarchy world of Sanctia evince a terrible new barbarism. They fall upon the planet's defenders and consume them bodily, devouring the living and the dead alike, as if compelled by a daemonic hunger. Bloating and swelling, the Orks become obese monstrosities that can move only at a snail's pace. Puzzled by this strange reprieve, the Adepta Sororitas systematically purge the Orks with flame and bolter. It is then that Mortarion and his Death Guard make planetfall. The skeletally thin Daemon Primarch looms over the bodies of the Orks as he stalks to the front lines. At his passing, each Ork bursts apart in a shower of foul fluids, and dozens of Nurglings spill out from their remains to follow their master. Sanctia falls to the Green Death within twenty hours of Mortarion's arrival, and the plague spreads across the Ecclesiarchy-held system at an unstoppable pace.

[...]

37th Millennium

312.M37 - 112.M38 The Abyssal Crusade

Warpstorm Dionys ripples across the galaxy, its ghastly energies polluting a wide swathe of star systems. In addition to billions of Imperial citizens, no less than thirty Space Marine Chapters are judged by the charismatic Saint Basillius to be tainted. The Ecclesiarchal lord gives a choice to those so judged - seek redemption in the Eye of Terror or be exterminated. Without exception, the Judged Chapters choose to embark upon a redemptive crusade aimed at purging the Daemon worlds of the Eye of Terror. Within that great Warp rift, some of the Chapters are lost forever or destroyed, and a few turn renegade in order to survive. The most resolute return to the Imperium nearly 800 years later, and discover that their accuser, Saint Basillius himself, is a servant of Chaos. The corrupted saint's demise is swift and bloody.

[...]

41st Millennium

[...]

757.M41 The Plague that Walks

The first outbreak of the zombie plague occurs on Hydra Minoris after Typhus, and his Death Guard foot soldiers, penetrate to the heart of its capital hive. As the living begin to fall prey to the painful disease, its true horror is revealed; the dead victims begin to rise up and attack the living. The resultant Imperial quarantine traps 23 billion uninfected citizens alongside a rising tide of the undead.

[...]

926.M41 The Vaxhallian Genocides

The vile Chaos renegades known as the Purge seek to vent their hatred of all living things, and choose the verdant Imperial world of Vaxhallia as their victim. The planet's surface is soon riddled with consumptive disease and crippling famine. Over the course of a single month, the Purge engineer the destruction of no fewer than fourteen billion Imperial citizens.

[...]

969.M41 The Terminus Est

The plagueship Terminus Est is sighted in the Cando System. It disappears soon after, but it is already too late. The zombie plague ravages all of the planets in the system over the following months, exposing the worst in human nature as brother turns against brother in their desperation to survive.