I have a sort of complex "magic" system all ready and polished (thanks to having it go through the wringer).
The idea is that there are two main forces that make up our universe. Darkness, which is more material, and Light, which is more energetic. Both are sort of ethereal on their own, not really tangible, but together they make what the world is made up of.
Now for every human being, there is a balance of this dark and light on a simple scale of 1-10 for each. If you are a 1-1, you are a normal person with an equal amount of dark and light inside you. That's the bare minimum. Of course, some people can be born with more than that, with either of those numbers being higher than 1, making them "special". Anyone within this criteria have the ability then to use this extra amount of darkness or light in many ways.
However, there is another side of this system. There are Dark Eaters, monsters that are made up of zero light, which make them unstable and barely existent. In order to remain existent, they must consume more and more darkness. They range from 0-3's to 0-10's being the strongest. These creatures eat the dark side of people, leaving only the light energy, thus they no longer exist on the physical plane, or simply, are no longer people.
So what does the world do to face off against these monsters?
The Brigade. A high-class of specials, all with anything more than the 1-1 levels. By using excess darkness to heal others and lure the Dark Eaters to hold them at bay, the excess light is used to "stabilize" the Dark Eaters, making them "equalized", and no longer a threat. Either they will disappear, or something else which I won't spoil here.
There are three main classes of the Brigade - the Dark Brigade, with excess darkness, are more about the defensive, mass, healing, trapping and luring. The Light Brigade, which excess light, are the extermination squad. They are more speedy, lightweight, ready to run in for the kill. Finally there's the Median Brigade, an elite of elite that have excess in both dark and light, who can use both sides and abilities.
Now the excess can be used as a power source, quantifiable by the simple number system. Everyone has a preset number when they are born, which cannot be changed no matter what. There is a lot outside this system to alter it, but I think I've already gone on for too long as it is.