The Skaven are a race of mutant humanoid rats living in a teeming society where overcrowded ness, the scarcity for food and the strict hierarchy based on the strong eating the weak (literally) make their existence very uncertain. Since ancient times the Skaven have searched the world for the stones of power that men in their ignorance call wyrdstone but which Skaven have long since known as warpstone. Wyrdstone is quite literally in their blood, for they feed upon it and make use of it in their foul sorceries. Now that the magical stone is in abundance in the ruins of Mordheim, the Skaven have send their warbands to secretly collect their bounty.

(Source: mordheimer.com)
More fluff