Skaven

"The sound of boots rang out from the flagstones of the Underway, the glowing lanterns to the side of the path barely holding back the oppressive darkness. A chittering whisper rang out from the darkness, the words a mockery of the dwarven tongue: "Must kill-stab man-things, yes-yes!". A tide of rat-men surged forward."
The skaven are a mysterious species who appear to be a cross between an overgrown rat and a humanoid, and are individually weaker than a human. They are completely unknown by the people of the Empire , and are obscure even among scholars versed in lore of the deep places. 
During the The War of the Broken Crypt, the War Council encountered a foraging band of skaven led by a pair of evil dwarves. During this battle, the party was nearly overwhelmed by swarms of skaven, as well as a monstrous beast nicknamed a "rat-ogre".  After the battle, it became apparent that the skaven were enslaved, that they spoke a mangled form of Khazak, and that the dwarves of Tznchundel had fought them long ago. They may have been under the sway of "the Deep Ones", though much remained unclear. When the Council returned to the surface, Baric of the Long Mind warned Garon against further investigating the Deep Ones. 
When the Army crossed through the Underway under the guidance of the War Council, the skaven were nowhere to be found. However, a few scouts sent into the northern reaches of the Underway never returned.