The Macharian Heresy was the hangover to the Macharian Crusade's raging house party. After Macharius's mysterious death following the end of his crusade, his old generals decided to fuck up the greatest Imperial campaign since the Great Crusade by trying to take the one thousand worlds they'd conquered for themselves, starting a civil war between each other and the Imperium.
When the High Lords got wind of what happened they sent in the Space Marines to get things back under control, with forces from nearly a hundred different Chapters being mobilized at the Heresy's highest point. It was during the Heresy that the Minotaurs first appeared, following all of the High Lord's orders without question, and nearly destroyed the Inceptors before robbing them blind. All in all it took around 70 years for the Imperium to end the Heresy and bring the worlds Macharius conquered back under control.