Shadowhunters, also known as Nephilim, are a secretive race of beings who are humans born with angelic blood. They are the appointed warriors on Earth of the Angel Raziel. They are appointed specifically to control and preside over the Shadow World, as well as over the demons and Downworlders who inhabit it.
Despite their ancestry, Shadowhunters are mortal and therefore vulnerable to old age and death; however, their angelic blood endows them with special abilities that allow them to achieve feats beyond humanly possible through the application of angelic runes.
The legend of the origin of Shadowhunters is that they were created more than a thousand years ago, when humans were being overrun by demon invasions from the other worlds. A warlock summoned the Angel Raziel, who mixed some of his own blood with the blood of men in a cup, and gave it to those men to drink. Those who drank the Angel's blood became Shadowhunters, as did their children and their children's children.
The most common tools of the Shadowhunter, and the source of their ability to fight the demonic Incursion at all, are the Marks of Raziel, a complex runic language given by the Angel to grant powers beyond mundanes. Some Marks are applied to the bodies of Shadowhunters, and some are applied to physical objects.