Skaven

Centuries ago, a magical plague sterilized most of the rat-like Skaven race in the Old War.  Long since though to be extinct, the Skaven did not go extinct though their numbers are a fraction of a fraction of what they once were.
The Skaven were driven figuratively and literally underground and many of the remaining survivors now dwell in the hidden catacombs of the West.
The other races of the West are not yet aware that the ratmen of Yesteryear scuttle among them, and the Skaven are eager to maintain the fiction that they no longer extinct.
There are a lot of mysterious disappearances out in the West afterall...