I noticed that The Weekly Wargamer posted the background for his army here and it inspired me to write down my own army's background that I've been working on. The army is Slaanesh daemons with IG allies. Without further ado, welcome to the dark side.
By coincidence or by the guiding hand of a dark god, an Imperial Guard troop transport made a stop at the hive world of Seputhule IV to resupply. Rushing between warzones, it wasn't until after the transport had left the planet that commanding officers noticed that a young sergeant named Charles Makris had gone missing. It's not unusual for soldiers to go AWOL in this manner, and turning back to search for the guardsman would have cost much more in resources than it was worth. Besides, on the massive hive world of Seputhule IV, he'd probably already completely disappeared into the teeming masses. Sergeant Makris would never be heard from again. At least that's what the commanding officers thought.
In fact, Makris had been corrupted during the company's last war, exterminating an alien race called the Bex. Unbeknownst to everyone but the highest ranking commanders, the Bex were actually servants of the Chaos Gods. During the final conflict, a Bex psyker had mind melded with Sergeant Makris, filling his head with forbidden knowledge and breaking his sanity in the process. Makris's condition took time to deteriorate, so he was able to slip by unnoticed until abandoning his old comrades at Seputhule IV. There, he began preach on the streets about "the realm that is seen without eyes". He was quickly branded a heretic, and was forced to flee to the Underhive. There, a member of an all female cult of Slaanesh worshipers called the Coven of True Joy heard his preaching and, realizing he was talking about the Warp, brought him back to the Coven. One of the Coven's psykers had recently had a dream that the Dark Prince was sending a prophet to deliver them from the Emperor's rule. The Coven quickly concluded that Makris was that prophet. They dropped their one gender rule, changed their name to the Church of Progression, and switched from disorganized, lazy hedonism to a plan of focused conquest. Makris brought two vital things to their plan, the training and combat expertise of the Imperial Guard, and knowledge of how to open a warp gate and summon daemons to aid them in their cause. This was the knowledge that the Bex psyker had bestowed upon him. The cult began aggressively recruiting outlaws and hive gangers, who were then given military training by Makris himself. Makris was so crazy by this point that half the time he thought he was still in the Guard and was training new recruits, but that didn't impede him. Before long, the Church had members in all levels of hive society, including the PDF. When the time was right, the Church assembled to overthrow the planet. Summoning daemons from the Warp to aid them, they seized the planetary armories in a well organized, joint strike. The PDF defenders who were used to facing unorganized mobs and gang members, were completely unprepared for such an assault. With the planet's supply of tanks and heavy weapons in their possession, the Church of Progression preceded to level the Arbites' towers and completely take control of the planet.
Imperial forces are currently en route to crush this heresy, but with the Church controlling Seputhule IV's powerful orbiting weapons platforms, getting a big enough ship within range to perform a simple exterminatus will be impossible. With daemons running rampant everywhere, retaking the weapons platforms and the planet will be a bloody battle. Meanwhile, the Church is hoping and plotting for a chance to seize the Imperial ships that are headed their way, so that they may spread the good word of the Dark Prince all across the galaxy...
Well that's it. Whaddaya think?