Skaven are creatures of Chaos, and generally not permitted within human lands. That Skaven
are even allowed as Heroes in these house rules is something of a stretch -- but one made
simply because some players actually enjoy the idea of playing oversized rats running around
with sharp pointy things. (Maybe it's just the excuse to get to talk in that absurd "rat
voice".) As always, such a choice is only permissable at the GM's discretion. (It's
certainly understandable if a GM familiar with the Warhammer setting thinks the very idea
of putting a Skaven in the party is beyond silly.)
That said, it might be conceivable that a Skaven might join the Heroes under extraordinary circumstances ... and any true Heroes will always find themselves in extraordinary circumstances now and then. Perhaps a Hero, in an amazing act of mercy, spared the life of a Skaven who possessed the rare ability to keep his word (or perhaps the Skaven in question simply deemed it more profitable to travel in the company of heavily armed warriors, rather than to trust his welfare to backstabbing fellow rat-men).
Origin of the Skaven
What happened to the Slann empire of the stars is unknown, but the Warp Gate was closed to travel between the worlds as it destabilized, and the poisoning, reality-warping effects of Chaos spread outward to engulf the arctic region.
The flows of Chaos are inextricably tied to the Winds of Magic, though the latter are more mild and controllable in their powers. When Chaos flows more freely, the Realms of Chaos creep slowly southward, and so do monsters and servants of Chaos ... while the Winds of Magic blow more strongly, and thus wizards are able to use more powerful spells.
Where Chaos is
strongest, it warps the very nature of reality, and the Realms of Chaos at the north pole are
far from being featureless plains of ice -- Rather, they are wastelands of madness, where
the landscape itself is alive and twisted into strange forms drawn from the nightmares of
Reality is malleable, and magic is so potent where Chaos flows freely that it is able to coalesce into material form. Most of this will melt away when the powers of magic wane again, but occasionally the powers of the Warp collect in small fragments of warped reality. Warpstone is an example of this -- a black stone that seems to draw light into it, surrounded by a haze of shadow, and cold to the touch.
Sometime in the distant past, in an age when Chaos flowed more freely, fragments of Warpstone were scattered amongst the lands of the Old World in some great cataclysm, and rats living underneath the cities of Mankind gnawed upon chunks of it. These rats were mutated by the concentrated powers of Warpstone, most of them poisoned and dying horribly, but some of them growing larger, more hardy, and more intelligent, and passing these traits to their even more mutated young. In time, the race of the Skaven was born of Chaos, combining the savagery of their rodent instincts with traces of intellect forming a dark parody of that of the humans whose cities they dwelt under. Even to this day, Warpstone is cherished by the Skaven, used to power infernal contraptions, and occasionally gnawed on by power-hungry sorcerers too maddened by ambition to be concerned about the dire risks they put themselves at by doing so, for but a short burst of increased magic.
Skaven are divided into many clans, and over the ages, many of them have risen in power, or fallen as rivals wiped them out. It should be noted that "clans" are not defined as purely familial ties -- If a Skaven is born to such a clan who fails to exhibit skills appropriate to his clan, he is likely to be cast out, or quite possibly killed outright. Conversely, Skaven who show special promise of interest to a particular clan are likely to be bought or stolen and indoctrinated in the clan's ways. Of special note:
From Clan Eshin comes the stealthy Gutter-Runners, who are skilled at using the networks of tunnels and sewers of the cities to great advantage in gaining surprise on their enemies. Even more skilled are the Assassins, and the truly gifted among them are accorded the title of Deathmaster. The emblem of Clan Eshin is a Skaven's claw clenched about a curved dagger poised to strike.
The average Skaven would not make a very good Hero, and a Skaven member of the average adventuring party would have a hard time of it, too. A Skaven Hero has to spend twice as much money for the purchase of equipment and services as a normal hero would. In abstract, this represents the various difficulties and hardships the Skaven faces in having to deal with the lands of Mankind -- or with his own kind, as a Skaven caught between two worlds, and forever wandering from place to place.
It is conceivable that a kindly fellow Hero might offer to purchase certain items for a Skaven character. This is wholly permissable, so long as it is an item equally usable by humans (or Elves or Dwarves) and Skaven. Please note, however, that items of armor (with the exception of shields) must be custom-made to accommodate the form of a Skaven, and therefore there is no way to get around the added cost. The same applies to services, which require the presence of the Skaven Hero.