Blood Contracts

Blood contracts are an old magic, but they're hard to learn and not many have the mind and will to learn them. It's an odd, finicky sort of working, legalistic and heavy-handed, full of lots of little fiddly bits that have to be lined up just so. It's exacting work, rather like enamel work or fine gemcutting. Unlike pretty enamel work, though, or fine cerameic tiles, at the end of an exhausting painstaking session of contract-threading one isn't left with something beautiful and a pleased client, but often a head-ache ad someone very cross that they've now been bound into something they can't get out of. So, not many people learn it, becaue it doean't make you popular to use it. But for those with the will and and the stomach to use it, who are willing to dispense with friendliness and make their way down other roads, magical contracts are indispensile. Once signed the contract sees itself kept through force. The ones who sign the contract cannot break it through fear of harm come to them if they become false.
Contracts being used for almost everything int he City, contract magic is almost universally useful. However, it's not learned by more than a scant handful of people beyond its most basic application, because of how finicky and exacting it is. It's more exhausting than most magic, and takes more concentration and focus than most to comoete. Also, most must keep up research of the law to apply that to their contracts as well. It's a ver rare few who can draw up magical contracts with the power to bind themselves into the blood from memory. These few have a talent much in demand, and able to command high commission without any trouble - though it might put a target upon them too, ffor many who've been bound into such contracts would go to dire lengths to be out of them, too.
The basic utility is very simple. The terms are laid out, and the marks of both parties are made. A little life is taken to seal the magic. There are specialties, in this arena. Blood is typical, or some emotion, but other things have bene used: sexual force, or memory, or tears. The magical force laid into the contract is used to punish its violation. They inflict punishment upon the blood, stealing it away or making it boil, filling it with poison, wicking away the life or stilling it until it sits sluggish and unmoving in the veins.  As horrifying as all this is, it's slow; always slow. And that's for a reason.  The justice of the contract always comes over time, to produce, one hopes, a change of heart. Or perhaps , its detractos suggest, the slow horror that befalls those wo violate blood contracts is simply the most vicious and visceral sort of deterantthere is, and plays itself out over days, weeks, months. Still, it plays to a purposee, either way.  This is telling: that's hte on e that can say of those who purpose this strnae and demanding art. They tend to be the sorts of people who never do anything without purpose. 
These are very old magics, very old and very powerful. Primal, not as base as some, but pulled from the belly of the world as some of those are and then rarified, sdistilled into the shaepe they are now.