Here is another one:
In the city of Asheril (the main city of my homebrew), undead creatures are not an uncommon sight. From well-placed and wealthy liches and vampires to municipal zombie street sweepers, a post-mortal existence is an accepted part of city life. However, one undead creature is constantly feared: the
ghoul.
Ravenous, relentless, and clever pack hunters, ghouls have threatened to devour the city on several occasions. An outbreak of a swarm usually begins with a single ghoul wandering into the city or more rarely one is created through
sorcery. Instinctively knowing that their strength lies in numbers, a single or small number of ghouls tend to go into hiding, quietly infecting those they find with ghoul fever and turning as many people as they can. At this stage of infection, they feast rarely and will flee if discovered.
Once their ranks have swollen to some threshold known only to themselves, or perhaps only till they can no longer contain their endless hunger, they burst forth from their hiding spaces and devour anything living they can find. Their hunger temporarily sated, they then tend to go back into hiding, but not before dragging off anyone still living that can be infected with ghoul fever. In dark holes and forgotten places, the next wave of ghouls is born, ready to repeat the process.
Only the most dedicated efforts by the city can break this cycle once it has reached a certain stage. There are few crimes more heinous than introducing a ghoul into the city (intentionally or otherwise) or worse creating a new one with magic.
The fear of ghouls in the city has recently been heightened, as a neighboring city-state has successfully weaponized the causative agent of ghoul fever. A virulent, communicable, air-born variant of the agent was first used approximately five years ago in a war with another group of city states. Deployed in a bombing raid in a major civilian area, the ghoul-bomb (as it has come to be known) caused a huge and almost instantaneous outbreak of ghouls. The city is haunted by its former citizens to this day.