The city has always had problems with the sewers, nests of rats, dead bodies being dumped in them, clogging and drainage problems. The Sewer guild, often near criminals themselves work ceaselessly to keep the drains open. Without them, the city would be knee deep in waste, human and otherwise. The city and its rulers pride themselves on the cleanliness and health of their city as it has been discovered that piles of garbage lead to disease.
When Sewer Guild workers turn up dead, savaged mauled and half eaten, or perhaps an errant limb washes out into a fishermans net, little notice is taken. They are only a step above criminals and some are criminals, working for the guild rather than languishing in a prison. Problems with the sewers increase as the guild is becoming hesitant to send men to their seeming death. Even armed sewer rats, as the guildsmen are called don’t come back, or come back half crazed with fear.
The city takes notice as waste starts piling up in the drains, overflowing into the streets. Then, some night folk, drunks and whore start vanishing. They have the same fate as the sewer rats, dead bloated with water, their limbs jerked out of joint and great chunks of flesh missing. Still no significant action is taken until a captain of the nightwatch is attacked on his patrol, leg ripped clean from his body. He dies but not before telling of some horror coming our of the drains.
The PCs are consulted to deal with the problem with sword and steel and spell if necessary. The city wants this handled quietly and as quickly as possible. The threat of a night hunter that rips its prey apart could severely cut back the number of travelers who come to the city, and thus take money out of the city coffers. They are willing to pay handsomely for concrete proof of the monster, and its demise.
Alligators in the Sewer - There is a lone alligator that has been living in the sewers unseen for a long time. something has eliminated the hunting in the lower part of the sewers (perhaps an effective anti-rat campaign) and the beast has moved up for better hunting. The challenge will come from very tight quarters, some tunnels requiring the PCs to crawl on hands and knees, possibly eliminating the ability to wear heavy plate armor, and the rot and stink of waste in the sewer. The hunt for the gator could be more perilous than the actual confrontation. Claustrophobia central!
In darkness Bred - Years ago, one of the Sewer Rats escaped, a mage of middling power. He ensorceled a baby gator he found, and it now does his errands, hunting his enemies and foes. He knows of the PCs coming to eliminate him and his pet. Does he prepare a trap for them, or does he make good a vanishing act, leaving them crawling in dank tunnels reeking of human waste?
Call Animal Control - A noble raised a gator as an exotic pet, but when he tired of the beast, he discarded it into the sewer drain. It was a burden to feed, and only lazed about. Now, driven by hunger and reptilian cunning the gator has shown the rest of the city that it will survive without its care. What do the PCs do when the monster has a collar and answers to its name? Is the lord responsible for the deaths inflected by his pet? Do the PCs blackmail him, turn him into the authorities? They can gain a great enemy this way if the lord can buy or talk his way out of the matter.