Magic is essentially the left over power of the old gods. Those who can use magic are able to draw their power from the realm of the Elder Gods to manipulate our world in a way they desire. They are a direct line to the Nether. Magic is dangerous however. Those who don't know how to use it properly run the risk of becoming possessed by daemons or even demigods, if their bodies don't burn up from their presence. Those who use it too much become more and more sensitive to the influence of the Elder Gods, often times losing their sanity. For that reason, the Church searches for those who are sensitive to magic and employ them in their efforts to hunt daemon kind and heretical activity, training them how to use their powers properly. There are only a handful of races however that are able to use magic, some are more sensitive than others. Those that aren't sensitive to magic aren't able to be possessed by daemons and can generally see through illusions. However, this doesn't just apply to magic users, this also applies to magical items. Magical items are conduits to the other realm, the more powerful the magic the more influence the item has on our world. The longer someone uses said item, the more sensitive they become to the influence of the Elder Gods, often times hearing voices from the other side. For some this is helpful, for most it's a hindrance.