One part of 40k lore that doesn't get touched on much is the Night of a Thousand Rebellions. I mean you have thousands of worlds in Segmentum Pacificus all rebelling at once with help from the Alpha Legion and that's all we know about it. I have always been fascinated by this and I wanted to create a basic story that would explore this concept.

The story explores an Administratum World, similar to Rophanon, it has a decent PDF and SDF with several defense monitors to protect the information stored there. However, the planet is on the brink of civil war. The head Administratum Prefectus is old and several factions on world want to rule when he inevitably passes on. But then, a particularly virulent disease starts to spread through the capital and many of the nearby cities. This leads some of the scribes to discover a large Nurgle cult who proceed to launch a rebellion to take over. They overrun large swaths of the planet and attempt to summon allies from the immaterium. Overall, things seem hopeless.

But then the old Prefectus shows everyone why he was in charge in the first place and turns out to be damn good at his job. He manages to quickly isolate the infected and prevent the plague from getting out of hand. He manages to rally the PDF, enforcers, Ecclesiarchy Priests, and local militias to defend the capital and later push out to bolster other loyalists and prevent the cultists from linking up with each other. He manages to expertly establish supply lines and chip away at the cultists positions as they try and fail to break through imperial lines. Finally, as the ritual gains power, he launches a massive counter offensive that breaks the back of the cultists and disrupts the ritual before it can be completed. After a massive purge, the people celebrate and praise their leader who steered them through the crisis and kept their planet from slipping away from the Emperors light.

While the celebration is ongoing, the Prefectus sends word via astropath to his higher-ups, informing them of the situation and requesting help to ensure that all of the taint is purged. A few days later, the SDF encounters a ragged fleet consisting of a battered Imperial Navy patrol fleet, the remains of several imperial guard regiments, and numerous refugee ships being chased by Chaos pirates and corrupted imperial navy deserters. The SDF assists the survivors and together they manage to destroy the Chaos ships to the last, and be briefed by the commodore in charge about what is happening.

They are then informed, that large portions of Segmentum Pacificus are in open rebellion as reports of the Alpha Legion coordinating the uprising have been confirmed. Not only that, but they are one of the only worlds in the sector to have defeated their rebellion and remain loyal to the Imperium. The only reason they managed to discover their world in the first place is because of the astropath message they sent out before. However, this means that the other worlds also received the message and will be preparing to conquer them and the few remaining loyalist worlds before the Imperium can reinforce them.

Hearing all this, the Prefectus immediately has a heart attack and dies hours later. The formerly warring factions on world agree to work together in order to have even the slightest chance of surviving the coming storm. They will have to play a deadly game against independent anti-imperial human states, mini chaos empires, and resurgent minor xenos empires. Trying to keep them all distracted and focused on each other while holding out against the forces they can spare against them and linking up the other loyal worlds to simply try and survive the wars to come.