Change [Book 1 of The Elemental Marks]

A @Fanfic Shadowhunters recommended story

A Mortal Instrument and Avatar the Last Airbender crossover understood by fans of one or both.

Control of the four elements, that is Air, Water, Earth and Fire, is a power mirrored only by the Downworld or by the Heavens themselves. Who but a warlock can hold a flame in his hand? Who but a vampire can move as fast as the wind?
And yet, Lina Bellewood is about to discover not only is it totally possible, but that the Clave has been running a program to train these benders, as they've come to call themselves. Even more frightening: Lina is one of them.
When the water rune is discovered on her and found to be there for years, Lina has to uncover the mystery behind it, and at the same time master waterbending. With a bender of each element by her side, they all need to find out what the truth is and what the Clave has been hiding. Is Lina going to be the bender who breaks the system or the one escape it?