City of Demons

Sam and Dean Winchester are on a break from hunting when their mini vacation gets ruined. Dean runs into Jace Herondale, who introduces them to the other Shadowhunters. They soon find out that there are major problems in the supernatural realm. Together they join forces to help save the world from destruction.
(This is a fanfiction with the characters from the tv show "Supernatural" and the book series "The Mortal Instruments".)