Once there was a renown school of magic. It was in a magical convergence and the school tunneled down into the mana-rich rock. Around the magical convergence was a Faerie convergence. (In my world, a convergence is a place where one of the Planes of Magic, of Divine, of Infernal and of Faerie intersect with the Mundane world. Travel between these convergences is limited but can be done. The really rare thing about this convergence is that the Magical Plane intersected with the Mundane, and around it was a second convergence, a Faerie convergence. This made the magic in the school very powerful indeed.)
The High Priest of Feneris, the Father of Evil, invaded the Faerie convergence and desecrated it, torturing the dryad who's home it was and leaving her nailed living to her tree. Since dryads only die when their tree is cut down and are otherwise immortal, this desecration warped the Faerie convergence and cut it off from the rest of Faerie. The wizards in the magical school went lethally insane.
One of the mages had several cages of weasels. She was trying to create a small, fast, agile and lethal creature that would obey her commands. However, unknown to her, the weasels' intelligence was magically enhanced due to the raw ambient magic of the place. The weasels, calling themselves the Ska'ag, or the People, escaped. They lurked and hid, learning of the ways of the ones they called the Makers. They learned of fire, of tools, of language and many other things. They also began to differentiate, a small group of them became known as the Shapers. They were the ones with the most magical potential in their bodies, and they could Shape stone, metal, and other solid objects, softening them to a claylike consistency so they can dig or shape them. But that came at a terrible cost. The lifespan of the ska'ag was only eight to ten years. When the Shapers worked their shaping, it drained their lifespan. Most Shapers died before their fifth or sixth year, wizened and enfeebled by their gift, but it did let them tunnel through the rocky walls of the school and shape bits of stolen metal into weapons.
When the Faerie convergence was warped into a Faerie/Infernal convergence, the mages went lethally insane and began to kill each other off. The Ska'ag watched in terror as mage-fires swept the halls, summoned monsters lurked and hunted, undead roamed the corridors and the fabric of reality was rent by the mages' spells. When it was over, all the Makers were dead.
Why were we spared, the Ska'ag wondered. No-one knew, although the best answer was that, since they were Awakened animals, and other animals were also unaffected, the Great Madness did not affect those of animal minds. Or perhaps the Source of Awareness to which they prayed decreed it thus. No-one knew.
The Ska'ag are weasel-like creatures standing about two feet tall on their hind legs, covered with a fine plush fur of white, grey or brown. Their only clothing are belts to hold their pouches and weapons. They are good swimmers and can hold their breath for several minutes. This is useful because the lower levels of the school are now flooded with water. They have prehensile paws that can hold tools and weapons. They cook their food, mostly over magical stones liberated from the Makers that radiate heat on command. They hunt rats and various other creatures for food and raise mushrooms. They reach maturity in one year and die of old age by nine or ten. Their eyes are nearly all pupil and they can see in nearly total darkness. To them, a candle flame would light a room, and most have never seen the sun. They can also see the BodyGlow of other living things (e.g. infrared vision). Their weapons are swords (read 'big knives'), small shields, spears and shortbows.
There are several hundred of them by now. They occupy the middle levels of the school, with the lower levels mostly flooded. They are working their way up towards the upper levels of the school, and some of the more adventurous scouts have actually been outside. The Shapers are working on eye coverings for the scouts when they go outside; so far, they have come up with goggles made of dark wood or stone with a tiny horizontal slit, much like the Eskimo use to prevent snow blindness which work adequately, turning daylight from intolerably, painfully bright to merely very bright.
Eventually someone (i.e. the PCs) will make it through the Faerie/Infernal convergence in an attempt to find out what happened to the school of magic, or to loot it. They will come across to the Ska'ags as things from their worst nightmares, more Makers. The Ska'ag can't stand up to the PCs, but the school is riddled with tunnels a foot across and the Ska'ag are good guerrillas. Hard to sleep wearing your armor.
Or perhaps the PCs can soak up the damage without retaliating long enough for the Ska'ag to realize that these are different from the Makers, for all that they look like them. One of the Ska'ag will venture out to talk to them. If convinced, the Ska'ag would make good guides and scouts, if they were properly rewarded. Lots of good role-playing potentials here.
Perhaps a curious mage, fishing around where the school used to be using Summoning spells, catches a Ska'ag. Does he kill it, capture it, or talk to it?