These monsters are the epitome of all that is vile. Meek and treacherous to the extreme, the Skaven would much prefer putting a knife in your back than to attempt a, straight-up, honest fight.
They have an insufferable need to aquire a strange and dangerous material called Wyrdstone. True to it's name it is a sickly glowing green rock that has mutating effects on a creature. Skaven use it for everything. From powering their weapons to shoot arcs of arcane energy, or transform beasts into the massively powerful Hell-pit abomination.