This is modified from my BFRPG supplement. Alternatively, you could use my skaven write-up for Swords & Wizardry White Box if you want.
Armor Class: 14
Hit Dice: 1 (see descriptions)
No. of Attacks: 1
Damage: 1d6 + poison
No. Appearing: 2d6
Save As: Normal Man
Treasure Type: L (C, O, Qx10, S in lair)
Ratman: 25 XP
Ratter: 100 XP
Brute: 145 XP
Sorcerer: 235 XP
Ratmen are humanoid rats, nearly man-size in height. They walk on their hind legs and they use their hands as would a human. They are typically brown or black, though whites are seen. Their eyes are a deep red and reflect light, making them appear to glow at night. It appears that their goal is the complete domination of the land and the slavery of all other races. As such, they have no allies among the other races.
All ratmen have darkvision. They speak their own ratman tongue and some (20%) speak Common. Ratmen also have the ability to speak with rats of any type, and often use rats as spies and forward lookouts. Ratmen are constantly exposed to disease, and thus gain a +4 on their saves when exposed to some form of plague. Regular rats will not attack ratmen unless magically compelled.
Ratmen weapons are poisoned. Anyone hit by one must Save vs. Poison or take an additional 1d6 points of damage.
The majority of ratmen have no special skills (~85%). They are usually encountered in packs of 2 to 12. They conform to the listed statistics, and have no special abilities. If ratmen are preparing for a major battle, they don chainmail and have an AC of 15. They wield spears and short swords in battle. They can muster hundreds of ratmen before a major battle.
Ratters are ratmen who have an innate ability to command rats, equivalent to a Charm Animal spell. They will be leading 4 - 16 giant rats when encountered. If in an area populated by rats, they can summon an additional giant rat each round along with engaging in normal combat. Ratters have 2 HD, with maximum hit points. They fight with a short sword in one hand and a whip in the other. They have only a -2 to hit with their second weapon (the whip). They Save as a Fighter 2.
Brutes are the ratmen warrior elite. They average 6 feet in height and are quite muscular (+1 to hit and damage with a weapon). They have 3 HD, with maximum hit points. They wear plate mail (AC 17) and wield two-handed swords, great axes, and halberds in combat. They are immune to fear (including spells, dragons, etc), and have a Morale of 10. They are often used as bodyguards by the clan elite, as well as being excellent shock troops in battle. Brutes typically aren't too bright (Int 7 - 8), and as such are rarely encountered in any type of leadership capacity (except for leading other Brutes). They Save as a Fighter 3.
Sorcerers are extremely rare. They have 3 HD, but cast spells as a 9th level magic-user. They have a bodyguard of 30 brutes with them at all times. They are the masters of the ratmen race, and all ratmen defer to them. Sorcerers maintain a library and a laboratory much as human mages do. Each Sorcerer has his own selection of spells, but they all tend to prefer spells that cause lots of damage (such as fireballs), as well as using curses and spells that cause disease or prolonged pain. They Save as a Magic User 9.
Ratmen are extremely territorial, and often war with each other. A different clan rules each area, and if members of another clan enter the area, it is considered an act of war. These wars are often very vicious, and are one of the main reasons the ratmen do not overrun everyone just by numbers.
Ratmen also fight amongst themselves to determine rank within the clan. These fights are seldom fatal, but all ratmen are scarred, with many missing an eye or ear. The Sorcerers encourage this behavior, as their god says that only the strong may rule. Sorcerers are above inter-clan warfare, and often visit each other. No ratman would dare assault a Sorcerer or his bodyguards.
Ratmen in Holystone
Ratmen have hundreds of tunnels built under the city, with many hidden entrances into the sewer systems. They have been known to raid into The Triangle District, taking people as slaves or worse.
Some ratmen scout the city nightly. They stick to alleys and rooftops, spying upon the city. They break into businesses and homes, stealing weapons, clothes, and food.
Ratmen are killed wherever they are found. The city offers a bounty of 1 silver piece per set of ratmen ears. Anyone who helps the ratmen is accused of treason and judged accordingly.
There is one ratmen clan controlling the tunnels beneath the city. Three Sorcerers rule them, one for the north, one for the area around the South Ward, and one for the area around The Triangle. Their reach extends out a couple miles from the city.
The other underworld races have to sneak through the ratmen tunnels in order to reach the city. Occasionally, a well-armed band of drow may travel through. The ratmen usually leave them alone, as the drow are too dangerous to face head-on when they travel in force.