Ecclesiarchial Wars

The Ecclesiarchial Wars was a colossal and destructive Religious Civil War fought between two major Ecclesiarchial sects that flourished within the Imperium of Man. Lasting more than two millenia, the war devastated the Imperium on a scale never seen before, even surpassing the devastation brought on by Horus' treachery. Like Horus Heresy and The Great Devouring, it left the Imperium broken and scarred, rendering it unable to mount any sort of counterattack as the Necrons and Tyranids seized most of the Ultima Segmentum. The Imperium barely managed to emerge from this destabilizing civil war and it was this war that prove the instability within the Ecclesiarchy and the Inquistion. 
In the end, a ceasefire was put in effect and the revolting parts of the Ecclesiarchy, referred to as the Redemptionaries, were given a place in the Holy Synod and various offices inside the Inquisiton. Recognized by the Coalition of the Benevolent Emperor, the ruling sect within the Ecclesiarchy, the Redemptionaries would eventually expand and replace the crumbling Coalition. Quadrillions upon quadrillions of souls were sacrificed in this massive conflict and it ultimately resulted with the Imperium of Man abandoning the entirety of Segmentum Ultima. Undergoing great reforms after this war, the Imperium became an even more totalitarian and oppressive nation, greatly limiting the power of many factions inside it and improving the authority of the Ecclesiarchy and the High Lords of Terra. The Inquisition was the most scarred faction of all, for this war nearly wiped out the organization entirely. Even though it started to rebuild immediately, the way the Inquistiors turned on each other and how they failed to avert this war caused many Inquisitors to question their philosophies. This war would eventually cause a sudden but subtle increase in Inquisitors following the Recongregator philosophy, most of these new Recongregators being veterans of the Ecclesiarchial Wars.