These monsters are the epitome of all that is vile. Skaven are a race of mutated humans that somehow fused with rats to become the warped beasts they are. 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 Warpstone. 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 guns to shoot arcs of this stuff or when used to combine beasts into the massively powerful Hell-pit abomination.