I want more of this. Reminds me on the anime Darker than Black, where those with power always had to fulfill some contract to use their power, else they'd die.
Man, this anime is so underrated. I like it a lot. I question the artistic direction on the second season, but at least the ending wrapped up the whole show nicely.
A little correction, nobody really know what happens when a contractor doesn't do that side-effect thingy. It is never mentioned if they would die, nor that it's even implied. The way I see it, they'd simply develop strong impulse to do so.