It is said that in the old days, when man still walked and had barely mastered fire, that magic was still kept away from him. The Gods did not want mankind's hubris to reach that of the elves, and as such magic and witchcraft was kept a secret. It was also at this time when the noble rat; intelligent, social, curious, and kindly was a creature loved by all animals in the Kingdom of Nature.
Seeing mankind's struggles in the dirt and briers, the kindly rat used its guile and small size to sneak past the guards of heaven and steal the secrets of magic from the Gods. They then took these secrets to man, and taught them to him. The rats, along with two other animals (ravens & cats), taught men how to hide, how to lie, and how to be silent to keep this knowledge secret.
As punishment, the Gods sent their creations and followers after them. The God of Wilds made birds of prey swoop down on rats in the fields, The God of the Hearth made cats seek and pounce on rats, and the God of Poisons and Secrets, so easily thwarted by the Rat, sent his serpents to strangle and crush the life out of them. The noble rat had been reduced to simple prey. Even the God of Mankind changed the view humans had towards the creatures, making them thought to carry the plague (this is false, everyone knows plagues are caused by miasma) and for ladies to scream and stomp on the tiny animals. This is the reason rats, despite their qualities, are reviled and spurned among all creatures of the Earth, and why only magic users and witches seem to enjoy and keep close these tiny furry friends. (along with ravens and cats, all creatures associated with Witchcraft).
Additionally, the intertwining of the working of magic and rats mean that among the most common types of mutations caused by magic are ones relating to rats. Whenever you get a rat mutation, roll on the table below. If you roll the same result three times, you turn into a rat permanently.
 Pervasive Stink. Your body produces a foul, musky odor unique to yourself. You get -1 to reaction checks if you do not wear perfume or if you haven't bathed today. If you get this a second time, your odor means you will always be at odds and will fight against any other magic user with their own odor, you have become a bit territorial, much like a rat.
 Rat Tail. Tiny vestigial tail appears just above your ass. It offers no benefits beyond being something you must hide. If you get this a second time, it elongates to several feet in length, becoming next to impossible to hide, but granting +1 to Climbing or Dexterity rolls if you have it out and trailing behind you and exposed to the wind. If someone cuts off this tail off, you lose -1d6 Dexterity permanently, but it does not grow back.
 Teeth. Your teeth start growing in your mouth, giving you an uncanny toothy smile. You need to constantly chew on things to keep them from growing too long. If you roll this a second time, your teeth begin to fall out and are replaced with rodent teeth; two long incisors and molars with a gap.
 Grooming. You must constantly be groomed or preen yourself. If you are ever dirty or otherwise unkept you get -1 to all saves vs magic and spellcasting rolls. You also start to produce prophyrin out of your eyes, which you must groom or you cannot see effectively each day. If you roll this a second time you start to grow a thin layer of fur around your chest, upper back, and belly and your neatness increases to a -2 to all magic rolls if you aren't fully groomed. You will need the help of others to keep your newly grown fur fully clear from lice or things caught within.
 Reproductive. Rats are known to have large litters, and your sexual urges increase. You will need to spend gold on a brothel or spend significant time with a lover every time you rest, or else you will become restless and aggressive, causing a -1 to all reaction checks. If you roll this a second time, whenever you have children they will always be a bit shorter then average but you'll have twins or triplets each time; many mouths to feed draining your wealth.
 Size. The first time you roll this, you lose -1d3 Strength permanently and become notably shorter. If you roll this again, lose another -1d3 Strength permanently, and your face and body has taken on a rather loathsome, ratlike apperance. You lose -1 to morale checks or saves vs supernatural fear. You also become afraid of cats, birds, and snakes.