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Faeries are fallen angels," said Dorothea, "cast down out of heaven for their pride." "That's the legend," Jace said. "It's also said that they're the offspring of demons and angels, which always seemed more likely to me. Good and evil, mixing together. Faeries are as beautiful as angels are supposed to be, but they have a lot of mischief and cruelty in them. And you'll notice most of them avoid midday sunlight—" "For the devil has no power," said Dorothea softly, as if she were reciting an old rhyme, "except in the dark.

Cassandra Clare (2011). “Cassandra Clare: The Mortal Instrument Series (4 books): City of Bones; City of Ashes; City of Glass; City of Fallen Angels”, p.102, Simon and Schuster
Faeries are fallen angels, said Dorothea, cast down out of heaven for their pride. That's the legend, Jace said. It's also said that they're the offspring of demons and angels, which always seemed more likely to me.