Our Worlds Collide (A fanfiction collage)

Thirteen-year olds, Rebecca and Ethan, were living normal lives until a note on Ethan's kitchen table. It warned them not to speak the words of an ancient spell. They have a drop-in look of their favorite fictional worlds. Will they survive the suddenly very real dangers? Will it change their view of worlds they always thought they worshiped, lands and characters they had fallen in love with?
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Rebecca felt the impact jar her bones again, felt the fall still vibrating her teeth even as she got off the floor. A few moments later, so did Ethan. Even through her clearing vision she knew exactly where she was.
Rebecca gasped. Ethan, also having read the same books as his best friend, quickly silenced her by putting his hand over her mouth. It was a sight to behold.
Rebecca's eyes were wide, taking in the details of the chamber. Once again she felt a numbing shock as her eyes took in the scenery hungrily, eating up every detail of everything in the room. The magnified worm-monster against one wall, and near it seven figures covered in dust as well as drying and fresh blood. Her eyes rose to the two humongous windows above on the far wall, displaying the conditions of two very different, yet connected worlds. One of dust, a land long dead. And the other, one where figures met death every moment. Two thrones forged from gold and ivory seemed to light up the whole room. There was no doubt in Rebecca's mind of what world they were in, especially once her eyes caught sight of the red headed petite girl poised and expressionless sitting on the throne to the right. Then she focused on a tall shape standing in the center of a circle that was bordered with letters, symbols rather, that we're covered in a substance that was undoubtedly blood. The figure was surrounded by a ring of blue flame, rising up from the mysterious language written on the floor.
He was a figure of dark beauty.

They had just dropped in on chapter 23 of: the City of Heavenly Fire. The Mortal Instruments