The Vermin evolved from the rats of Dereth, twisted into their current forms by the Devastation (which also created the Nemeses and such other races as the Sclavus, Mimbu, and Darkenfowl).
The Vermin are omnivorous scavengers who prefer the dark of night to cover their activities. Since they often compete for the same food sources, the Vermin maintain rivalry with the Grutts; in their simpleminded way, Vermin consider the Grutts to be unclean. Vermin typically travel in gangs of two to five in size. For the most part, they are dangerous only to the least experienced adventurers, although Vermin leaders (known as 'kings') can be more formidable enemies.