S&S (d20) - VIGIL WATCH: WARRENS OF THE RATMEN (2002)

From the back of the book:

Lurkers in Shadow

The Ratmen - or, as they call themselves, the slitheren - are some of the deadliest foes adventurers can face: foul humanoid rodents, intent on spreading evil throughout the lands of men, elves, and dwarves. In the dank reaches of the Mourning Marshes, the many broods of ratmen have come together in a profane alliance, intent on mayhem and the downfall of all other races.

Vigil Watch: Warrens of the Ratmen: includes a wealth of information about the ratmen, their culture, society, and biology. Here, you will read of their diabolical plots and schemes, of the twisted monsters that lurk in the deadly marshlands. Here, too, is information on new broods of slitheren - the sorcerous Black Pelts, the powerful Storm Chasers, the cunning Forge Crawlers, and the enigmatic Unseeing, as well as rules for developing slitheren as player characters. Two detailed ratman settlements come ready made for adventuring.

Join the Struggle - meet the slitheren and their minions face to face. And pray that you live to tell the tale.