Jace Herondale: You asked me what happens when you carve Marks onto someone who doesn't have Shadowhunter blood. Just one Mark will only burn you, but a lot of Marks, powerful ones? Carved into the flesh of a totally ordinary human being with no trace of Shadowhunter ancestry? You get this. The runes are agonizingly painful. The Marked ones go insane, the pain drives them out of their minds. They become fierce, mindless killers. They don't sleep or eat unless you make them, and they die, usually quickly. Runes have great power and can be used to do great good—but they can be used for evil. The Forsaken are evil.

Clary Fairchild: But why would anyone do that to themselves?

Jace Herondale: Nobody would. It's something that gets done to them. By a warlock, maybe, some Downworlder gone bad. The Forsaken are loyal to the one who Marked them, and they're fierce killers. They can obey simple commands, too. It's like having a—a slave army.