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1.
An elite group of fictional warriors (from the City of Bones books)
Trained since childhood to fight and slay the downworlders, demons and faeries that attempt to create havoc on earth.

Shadowhunters are different from humans which they call "mundanes", the Shadowhunters are what you could call an enhanced race of humans

Shadowhunters are physically marked with Runes over their skin, the older or more elite a Shadowhunter, the more Runes they will have

The Runes create different types of effects, some are defensive and some aid strength etc.
these Runes are tattooed onto the Shadowhunter using a Stele (a specially crafted Shadowhunter weapon)

Its a really cool concept so go read Cassandra Clare's books:
City of Bones
City of Ashes &
City of Glass
And Yes, I do have way too much time on my hands :)
"It means 'Shadowhunters: Looking Better in Black Than the Widows of our Enemies Since 1234.'" -- Jace Wayland (a Shadowhunter from the City of Bones books)
by Skins4Love June 17, 2009
 
2.
Kickass demon-killers from Cassandra Clare's series: Infernal Devices and the Mortal Instruments.

These demon-killers protect all mundanes (humans) from evil demons who want to wreck havoc in the world.
-Did you see me kick that demon's ass?
-Damn, dude, your one out of the group of Shadowhunters I know that would do THAT to a demon.
by Lurien February 23, 2011