Just dropping in and leaving something. It was not originally intended for public consumption, but since I've got it I might as well post it. Likely too late to really do anything, but who cares?
Malignant assistance
In the aftermath of the second incursion, many among the citizens of Avernus succumbed to promises of power, revenge on the unfair life and world that tormented them day to day and fulfilment in the name of the blood god. And in the wake of his warriors, other cults saw their chance to rise. Soon, numerous cults grew among the population like festering tumours, and once again Avernus waged war on its own.
Wizened by past experience, they send out not just the hardened Arbites, but also the soldiers that had only recently fought the daemons of the incursion, and the witch hunters. Their duty was solemn, but they were prepared to do what was necessary. Among them, the living legends of Avernus were sent out, to lead through both might and mind. Even the creatures of the Hellworld, decimated by the Incursion, were seen occasionally moving with strange purpose against targets that would later turn out to be servants of Chaos.
Still, the memory of the crimson skies was seared into the memory of the populace, and many could not resist the call of Chaos. Tens of millions rose against their protectors, and soon the Hives of Avernus were stained in blood.
But, among the cults, another force grew. Like a tumour within a tumour, there were those enraptured by chaos, but hateful in their hearts towards the ruinous powers that had stolen their lives. And so, within the cults, some turned again, and found another force that guided them, empowered them to unleash their hatred towards what they had become. Their numbers were few, but rumour of their acts soon spread among the cults, fuelled by terror.
Those who revelled in the name of she who thirsts were left with their nerves scorched by sorcery, unable to feel anything. Those following the abomination were left scattered by unknown attackers, isolated from the ones to order them and driven towards the Arbites with their minds and cohesion broken. Those who bore foul plagues found themselves healed, only to be executed soon after, promises of eternity in solitude accompanying them into the eternal beyond. The warriors of the blood god were hit hardest in retribution for their connection to the root of the incursion. They were crippled by the dozens, left to be found by the authorities unable to move as much as a muscle, begging for a chance to die fighting and being denied it.
Even in the great uprisings, the traitors were among the cults. Rituals were disrupted as the sacrifices turned on the sorcerers; formations broke as soldiers turned on their brethren, not even bothering to evade fire as they killed as many as they could. One Arbite still tells of storming a command post, only to find an acolyte standing over the cooling carcass of her former leader, taking care to put eleven shots into his back before turning towards the Arbites, smiling as she received death.
Of course, rumours spread, trying to make sense of these occurrences, but they were few and clueless. No Avernite would willingly believe in something this convenient happening without suspecting some kind of trap, but none could find any sense in the twice-betrayers actions. Many decided to just attribute it to the usual madness of Chaos, and in a sense they were right.
In the aftermath of the madness of the cult outbreak, not much is to be learned from this. Some claimed the planet had revealed another of its many weapons, others speculate Avernus had drawn the attention of another ruinous force, willing to exterminate the competition before turning on the civilians as all of them do. The only ones who found a hint of the truth were those that had served for a long time and had witnessed something similar happening in a past campaign.
But in the end, the only ones able to answer this question took the answer with them as they passed by the hands of their brethren, and sometimes their own.
I'm supposed to tag this
@Durin , right?
( inspired by some Malal fluff)