I've also been working on a little bit of fluff to give background to the campaign. This isn't meant to be a "story write", but more me getting down the very basics of why there is a conflict. We quite liked the idea of reversing the usual "Chaos incursion - Imperial defence", as well as trying to paint more complex picture of Nurgle devotees. However, I'm not that accomplished a writer of chaos lore (or in general, lets be honest) so if anything is wildly off-canon, let me know.
I drew a lot of inspiration from Dan Abnett's description of worlds under chaos control. They do function, there's just a lot of chaos worshipping going on! And Nurgle is often painted as a kindly grandfather, as well as the god of rot, disease etc.
Before the Nubes Agonia
Cyt Sardus was a shrine world on the far southern fringe of the Sutaris Sector. It was considered the holiest place of the entire sector, and enshrined in it was at least one reliquary for each of the sector’s saints. Every saint had a shrine city built around their bones, or a particular sacred relic. The greatest city, Nastoriam, was built around a reliquary of a near complete skeleton of Saint Nastor the Relentless, and a few preserved vials of his holy blood. The entire planet was built to serve the many pilgrims who flooded to the planet in their billions, and to protect the relics from invasion.
Of course, such services came at a price. The Citadel of Saint Ayr the All Conquering was particularly lucrative for the Ecclesiarchy and merchants, as the Saint had decreed it a holy duty to his followers to make the pilgrimage to his tomb in their lifetime. As only a single hand remained of Saint Ayr (following his Holy Martyrdom under the foot of a Stompa titan as he inspired his men to a good breakfast), he was a relatively low maintenance Saint too.
The Descent of the Nubes Agonia
In 747.M41, the southern reaches of the Sutaris Sector became engulfed in a local warp storm. The warp storm was nothing large, by galactic standards, but it still cut off Imperial communication to a dozen systems. The storm also blinded the astropaths the ships relied on to travel through the warp, meaning they were piloting blind. One of the systems most severely cut off was Cyt.
But for countless Imperial citizens, their pilgrimage had already started. Many ships tried in vain to reach the system through the warp storm. Pilgrim ships, poorly staffed and maintained at the best of times, were engulfed in their thousands as they tried to navigate without the light of the Astronomicon. The demons of the warp feasted on millions of souls, until the Imperial Navy prohibited all warp travel into what would become known as the Nubes Agonia.
Cyt Sardus Falls
Cut off from the rest of the Imperium, and without legions of pilgrims to fuel their opulence with indulgence money, the upper classes of Cyt Sardus fell into disarray and infighting. Cities declared war on each other and whole regiments of Shrine Guard, soldiers whose only purpose was to defend the reliquaries, were instead mobilized against each other. Ulstoriam, dedicated to Saint Ulstor the Unifier, was destroyed after just four years of isolation. A hundred thousand pilgrims, stranded in the city by the warp storm, died at the siege.
And all the time, Chaos whispered to any who would listen. As the wars became longer, and the death counts soared, so too did those willing to hear the offer of Chaos. Pilgrims who had travelled far to worship at the feet of the Emperor’s Saints instead offered themselves to the Great Enemy. Priests stopped preaching the holy word of the God-Emperor in grand cathedrals, and disappeared to preach the word of Khorne, Tzeentch, Slaanesh and Nurgle in secret locations. Less than a decade after the Nubes Agonis had fallen, the many disparate cults to Chaos on this once holy world posed a serious threat to the remaining Imperials Shrineguard.
When it finally came time for Chaos to rise, it was swift and against little resistance. The Shrine Guard meant to oppose them were either corrupted already, or so weak from war that what few loyalist regiments remain simply crumbled. Ecclesiarchs pledged to Chaos slaughtered their brothers to make way for new gospels.
Even as the last Imperial forces were being destroyed, the Chaos cults turned on each other as well. The cults of Khorne, Slaanesh and Chaos Undivided were the weakest, and the first to go. For 5 years the cults of Nurgle and Tzeentch fought for control of the planet. When the devotees of Nurgle finally triumphed over their trickster brethren through sheer perseverance and single mindedness, they had formed into a fully organized and battle hardened army. Nurgle Now, they could shape Cyt Sardus how they wanted.
The Putrid Pearl of Nurgle
The Nurgle cults of the insurrection grew to be full religions, each venerating different aspects of the God of Decay. The Entropists worshipped His reassurance that everything, eventually, would fall into disorder. The Pustain Brotherhood devoted themselves to discovering all the maladies He could offer and inflicting them on each other. The Necrotants channeled His will into raising the dead, to form zombie slaves.
One of the most popular temples was that of Vitalants, who worshipped Nurgle’s ability to give life. To see Nurgle as a god of decay, they argued, was to put too much emphasis on human beings. From the perspective of microorganisms, Nurgle was indeed a god of life. They preached the longevity that Papa Nurgle could bring them, and to recognize their place as a habitat for all the many bacteria, viruses and funguses He had brought into the galaxy.
New governments sprung up alongside the temples, focused on each of the many shrine cities. Some would confederate into nations, whilst others would remain independent or grow into states themselves. The new leaders, most elected by the common man for all would rot and so all were equal in the eyes of Nurgle, set about improving the infrastructure of their nations. Cyt Sardus had never been well equipped for agriculture, relying on imports that they bought with pilgrim’s coin. In the years of war after the Nubes Agonis fell, millions had died of starvation as the planet’s stocks rapidly dwindled. Whilst some attempt at agriculture had been made, too much of the planet was considered holy for the Imperial authorities to countenance turning the land to agriculture or industry.
The new leaders had no such qualms, and ancient gardens and vistas dedicated to the Saints were bulldozed to make way for fields and factories. Nurgle blessed His children with a great many giant funguses, which could grow in Cyt Sardus’ poor soil, and soon famine became a thing of the past. Rapid industrialization, aided by defectors from the planet’s limited Adeptus Mechanicus presences, meant each year the crops grew and the labour needed form the workers decreased.
Crucially, arms factories were also developed. Whilst the Nubes Agonis protected them for now, many of the new leaders were cautious that it might not stay forever. If the Imperial forces were to invade, they wanted to be prepared. Though the planet was split into nations, which sometimes came into conflict, they were all prepared to be united against any Imperial attempt to reclaim the planet. As it happened, another threat would appear to test their unity long before any Imperial forces arrived.
The Tzeentch Incursions
In a tiny corner of his vast, incomprehensible and incorporeal mind, Tzeentch was slighted at the victory of his brother god. The conquest of a planet was mostly done simply to amuse him, but Cyt Sardus represented something a little more. The chance to take an Imperial prize and twist it to his liking. Tzeentch objected to Nurgle, in his grandfatherly way, instead taking to nurturing his new wards and letting them construct a better world for themselves.
In a time span that the humans on Cyt Sardus would recognize as fifty years, Tzeentch invaded eleven times. Each time, he would send down legions of demons and mortals pledged to him. Each time, he was repulsed by the devoted and dogmatic legions of Cyt Sardus. Eventually, Tzeentch would try a new tactic.
Pulling back the Curtain
Tzeentch manipulated the tides of the warp so that the Nimbus Agonis would clear just enough to let the Imperials reach Cyt Sardus. The Imperials would seize the chance to retake Cyt Sardus, Tzeentch correctly reasoned. Either he would take it from them, or at least Nurgle would be denied his new prize. And as the Imperial forces emerged from the warp to retake their Shrine world, Tzeentch waited and watched to see his brother’s next move…
 See Jonn Hermbrecht’s The Conquests and Most Holy Martyrdom of Saint Ayr for more details of the foul greenskin ploy that Saint Ayr most certainly knew about, and allowed to happen anyway as that was what the Emperor decreed.