Shadowhunters don't just rely on the stele, a magic marker (emphasis on the magic) for drawing tattoos on their own skin. They can also cause real harm with magical seraph blades that look like harmless wand-shaped steel tubes until a blade springs out of it like a beam from a lightsaber.
Jace explains to Clary that they're called "Sanvi, Sansanvi, and Semangelaf" (6.5), but he doesn't elaborate any further. Thanks, dude!
We do get to see Sansanvi in action when he raises the silver wand-shaped object into the air like he's Sailor Moon or something and shouts out its name. A blade emerges from the tube, "clear as glass, with a glowing hilt, wickedly sharp and nearly as long as Jace's forearm" (6.82). Yowza.
Here's the skinny: Sanvi and Sansanvi are angels who tried to bring Lilith back to Eden where she belonged. But she wasn't having it, so the angels were rebuffed. Semangelar is also one of these angels, but it is also invoked to assist pregnant women. Um, we don't want to see what Jace is going to do with that wand.
Wait a second. Who is this Lilith lady? No, not Bebe Neuwirth from Cheers and Frasier. According to some traditions, Lilith was Adam's first wife, before Eve came along. According to other traditions, she was an evil she-demon who strangled babies and seduced innocent men. You decide.
While we're on this (un)holy subject, let's talk about the Mortal Instruments themselves, shall we? There are three that we know of: a cup, a sword, and a mirror. The Mortal Cup plays pretty heavily into the narrative taking place in City of Bones, and it kind of reminds us of the Holy Grail.
The legend goes that "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" (5.238). Easy enough, right? Wrong.
Not all who drink are worthy: "The reason that only a few humans are selected to be turned into Nephilim is that most would never survive the transition" (10.56). Eep. We hope they don't end up like that old dude at the end of The Last Crusade.