Liber Mutatis indicates that in 2518, Auerswald suffered a daemonic incursion in which horrors roamed the streets mutating all they touched, Imperial army took 2 weeks to cleanse streets and priests a year to cleanse taint etc. (to 2519). This was the result of Tzeentch’s cult’s efforts.
Assuming a campaign start date of 2521 (see Eye for an Eye, page 71 for assumed start date of 2521 if you do the math of 2515+5 years +1 year later = “now” for the start of a campaign), this overlaps with the period “a few years ago” when Count von Jungfreud provoked issues with Auerswald and was hobbled by Ubersreik being made a free state, according to Edge of Night .
Note Auerswald is referred to as a free town in some earlier edition sources.
Weaving this together and telling it from a "von Jungfreud perspective", as safe to publicly state in the area:
When the free town of Auerswald suffered a chaos incursion as a result of ritual by cultists of Tzeentch, many mutants were created and many scattered into the woods and area nearby. Some even sought to remain and carry on their lives (doubtless with evil intent) and had to be rooted out.
Count von Jungfreud significantly increased his troops at the border with Auerswald and area to catch mutants and prevent them coming up-river into his territory to plague his people - as his duty to the Empire and those under his protection required. This meant checking travellers in case they were mutants hiding their true natures. He enlisted priests of Sigmar and even a Witch Hunter in this endeavour.
Of course, this laborious task had to be funded by additional tolls and tariffs. Furthermore, sometimes a Sigmarite zealot or Witch Hunter would head off in “hot pursuit” of a fleeing suspect, going out of the Count’s territory, and the Count’s soldiers (devout and brave as they are) would follow after to assist even if this took them into Auerswald’s streets.
Count von Jungfreud denies this was a “power grab” or attempt to usurp the free town of Auerswald ’s status as a free town (pulling off what a Countess of Averland infamously did a few years ago when the free town of Streissen in her province ran into trouble). Such accusations are most hurtful to the Count, particularly those that accuse him of actually turning back mutants into Auerswald to foment discord in the free town - this is outright slander and he would challenge any noble that spoke it. However, Imperial politics being what they are, he was cast as an ambitious expansionist, seeking to seize territory while the Auerswald town council and militia were still weakened. Other nobles sent soldiers to take over guarding the frontier and the Emperor “invited the Count to withdraw” and also granted Uberseik its charter.
Many Sigmarites and at least one Witch Hunter are still very keen on the Count as a “the sort of strong, no nonsense leader needed to deal with the threats of chaos”.
Whether some part of this was to further a failed or still brewing scheme of Tzeentch remains to be seen. Was Tzeentch hoping von Jungfreud would provoke civil war and weaken the Empire (what almost happened)? Is weakening von Jungfreud the real objective (what has happened so far)? Is it something else yet to happen such as a new ruler of Ubersreik...
Any thoughts/suggestions on how this can weave together welcomed. I'm doing this not so much as "here's a grand plot" but rather as "here's a situation you can fold into a grand plot to suit you."