Pestilence
There's a species of Grimm that resides in a coastal city within the Kingdom of Vale. This city is one of the smaller cities that lack the political or economic clout that the larger city of Vale has. Years of Grimm attacks and emigration have lead it to being a rundown settlement populated by people who dream of leaving one day.
Anybody visiting the city will immediately notice something off about it. It only takes one careless glance to notice that one half of the city is actually populated while the other is completely empty. The reason for this is an unfortunate one.
Many years ago, a Grimm found its home in the city. This is the Grimm known as pestilence. It feeds off of disease, growing in size and lethality with every bit of exposure to diseases present. This all started when a plague passed through the city. It came off the back off the Grimm when it first arrived, resembling a slimy tumor covered in cists. This Grimm breathes through the multiple little holes in its skin, releasing contagious smog of disease. Skilled Huntsmen know to attack this Grimm from a distance with gear that allows them to avoid breathing in its smog. Most ordinary people within the city, however, are not Huntsmen and didn't know better as they carefully observed the repulsive blob when it first showed up.
After a while, people began to cough and experience gut-wrenching pain. This eventually spread to people they knew, and then to a large chunk of the city. A quarantine of the city eventually took place that saw families torn apart and many people dying as they became trapped with the incurable disease. Since then, one half of the city serves as a reminder to the threat of Pestilence. It no longer resembles the slimy little tumor. The Grimm has long since fed off of the disease that it's created and is now much more intimidating.
The Grimm now looks more like a diseased humanoid. It stands at about half the size of a large building and tends to stick itself to the walls of abandoned buildings. Its large head has an entire side that's riddled with holes. Maggots infest these holes, dropping out of them every so often as the Grimm moans as if they give it pain. Large tumors cover its back with bulbous pimples that decorate them like sprinkles. The top of its head is covered with matted hair. Disembodies body parts can be found inside of its hair, most likely from people the Grimm has killed.
The people of the city have a border that's meant to keep them from going anywhere near the city, but those same people also with to be free from the presence of Pestilence.
At this moment, four young Huntresses find themselves approaching the city. None of them know about the city's history and none of them are prepared for what they're about to encounter.