Though physically weaker than most adult humans, the Children of the Horned Rat, or skaven as they are more commonly called, are naturally faster, more agile, and they are natural miners as well. Mystery shrouds the origins of the skaven. These creatures do not have a history. Their past as a race holds no interest for the skaven. They concern themselves only with two periods of time: the present, during which time they do not rule all, and the future, during which time they shall rule all.
Various other races have compiled histories of the skaven, but except for those related to military conflicts, there seems to be little agreement. Some histories claim the skaven are degenerate humans (or elves or dwarves or halflings). Others claim the skaven originated on another plane of existence, or that they were specifically created by the Horned Rat from common rodents. Whatever the truth, this much is certain: The skaven are a blight upon the world. They worship disease, poison, death, and conquest. All other races are their enemies, including those with which the skaven may enter in alliance with.
The most commonly encountered skaven are slave rats and clanrats and their military superiors, the stormvermin and warlords. Each clan typically includes several dozen clanrats, a few dozen stormvermin, and, at most, two or three warlords. The slave rat population flucuates due to pogroms, desertions, murders, et cetera, but most clans have many dozen slave rats scurrying about, many of which live lives similar to feral dogs.
Armor Class: 8 
Hit Dice: 1/2
Attacks: Weapon (1d6-1)
HDE/XP: < 1/10
Slave rats are the lowest members of skaven society. They are treated little better than stray dogs at best. Slave rats are the weakest, smallest skaven, usually set aside at birth as being the runts of their litters. Slave rats seem to be invariably stupid, cowardly, treacherous, and greedy. Skaven of other classes may kill slave rats with impunity, although those slave rats owned by a master may object to the loss of property.
Armor Class: 6 
Hit Dice: 1-1
HDE/XP: < 1/10
Clanrats are the most numerous type of skaven, superior in rank only the slave rats. As their name implies, clanrats belong to specific clans, and they serve those clans in all but the most menial capacities. In times of war, clanrats make up the bulk of common troops sent into battle in order to soften up the enemy for more elite soldiers such as the stormvermin.
Armor Class: 5 
Hit Dice: 1+1
Stormvermin are the elite warriors of skaven society. They make up the rank-and-file of the military caste. Stormvermin are nearly as tall as humans, and they can be fearsome opponents on the field of battle as compared to many common soldiers.
Armor Class: 5 
Hit Dice: 3+1
Special: +2 to attack rolls, inspire skaven
Skaven warlords are the largest, strongest, and most vicious of the military castes. Their presence on the field of battle inspires lesser skaven to frenzied acts of violence (+1 to attack and damage rolls to skaven within 60 feet who can see and/or hear the warlord). A skaven warlord typically commands a couple dozen stormvermin, several dozen clanrats, and however many slave rats can be rounded up and driven toward the enemy.