Death of Innocence
It occurred when Fabricator General Kelbor-Hal made an alliance with Warmaster Horus Lupercal through his agent Adept Regulus. As part of the pact, the allies of Kelbor-Hal were to openly turn against the loyalists of the Emperor in order to prevent Mars's assets from going to the enemy. As an incentive, the Warmaster gave Regulus the means to open the Vaults of Moravec which released a corrupt scrapcode that spread through the digital mechanical network on Mars leading to a great deal of unrest and devastation as well as the secret rise of the Dark Mechanicum.
The Death of Innocence, as it was later known, led to numerous facilities being destroyed, from power plants to factories, as well as the deaths of a large number of Astropaths on Mars when their life support systems were poisoned by the corrupt scrapcode. Officially, few knew who perpetrated the attack at the time and members of the Dark Mechanicum attempted to get the support of other techpriests by telling them that the event was caused by the Emperor, who sought to conquer Mars now that the Great Crusade was nearing completion.