Magic is a tangible force in The Dark Mod setting. Most people encounter it in a subtle form on a regular basis. Even more so than in our own history, there is no clear line between "magic" and "science"--most people see alchemy, electricity, and mathematics as types of magic (or magic as just another form of science, if you prefer).
The more overt or "flashy" the magic is, the rarer it tends to be. Minor potions or wards against rats are common. Shapeshifters or fire-throwing mages are much rarer.
While anyone can learn alchemy or mathematics, sorcery is an innate talent. Most people cannot do it. Those that can often learn, though trial and error, to do fairly easy things, like using tea leaves to tell fortunes, heal minor aches and pains, or creating small wards against rats or disease. With a little knowledge of alchemy or herbalism, these people can create successful potions or amulets.
It is not enough to have a strong natural affinity for magic (though mages must have this as well). To learn mastery over the elements requires intensive study and training. Trained Mages are able to do much more powerful things than the local wise woman or herbalist. They can summon storms, hurl balls of flame or ice, and create illusions to mislead people. Some can even communicate with the spirit world and summon them to do their bidding.
Every time a mage casts a spell there is a chance, however small, that the mage can lose control of the magic, causing unforseen and potentially disasterous effects. One of those effects is aging unnaturally. Other effects depend on what the failed spell was trying to do. A miscast fireball might cause the mage to set fire to the hair of all bystanders, or to spontaneously combust. The more powerful the spell, the more dangerous an accident is. That keeps mages from using magic frivolously. It also provides all kinds of interesting story ideas, as a miscast spell could result in a demon running wild through the city, or a powerful magic text being teleported deep underground where only a master thief can retrieve it.
Pagans are very comfortable with natural magic. It is common for pagans to practice simple rituals and use talismens to attract good fortune. Their tribes frequently have shamans or druids who can conduct rituals to create more powerful magical effects, such as shapeshifting or summoning animals. Pagans don't study books or academic magic like Mages, however.
The Builders have priests who can create magic-like effects, such as throwing flame or summoning light. These effects are said to come directly from God, however, and are therefore considered holy miracles, not magic. Likewise, common people believe in the magical effects of prayer and the use of holy talismens, like the bones of saints.
Necromancers study a more macabre form of black magic. They are interested in learning about the forces of life and death. They learn by studying anatomy, both living and dead, and how bodies decay after death. As they gain power, they learn to control life forces, becoming able to animate the dead and extend their own life. Necromancy is highly illegal in the city, and its study tends to corrupt the necromancer, twisting his limbs and rotting his skin. Eventually necromancers start to resemble the corpses they dissect.
Necromancers often put animal heads on pikes in their territory. Using their magic they can animate the heads, making gruesome sentries.