Vaxhallian Genocide

The Vaxhallian Genocide is what happened in 926.M41.During this cultists of The Purge attacked the planet of Vaxhallia, infecting its inhabitants with deadly disease and destroying their crops, leading to famine.

These actions have led to the loss of over 14 billion citizens of the Imperium.

Taking Action
The Imperium acted quickly, for Vaxhallia was an Astropathic Relay Hub and a Fortress-Planet, very important to the defence of the Herakles system.

They sent out the entire Hammers of Dorn, Fire Lords and Crimson Fists Chapters to begin planetfall, accompanied by 12 Cadian Battle Groups along with Imperial Knights of House Raven.

Vaxhallia soon becomes a flaming battefield.

The Coming of Typhus

Things become more dire, for the unholy Terminus Est jumped out of the Warp near Vaxhallia, flooding the planet with Plague Zombies and Plague Marines.The Cadian forces fall to the flesh-consuming Zombies, filling their unclean ranks, with Typhus leading the undead legions and the cultist mobs towards the Space Marines and the Imperial Knights.

The second Battlefleet of the Imperium came,yet its Commander believed the world to be irrevocably lost.

The evacuation of healthy survivors began and, as that ended, the world was burned and turned to ash due to an Exterminatus issued by the Commander of the Battlefleet.

Sources:
Dark Millenium,7th Edition