The Hound of the MacLeods - Chapter 1 - BluePhoenix73 - Supernatural/Castle [Archive of Our Own]
The supernatural doesn't really exist in Kate Beckett and Richard Castle's New York, so when a hellhound gets loose and kills someone in Manhattan, Crowley, the King of Hell himself, calls on Sam and Dean Winchester to track it down and bring it back to him. A reinterpretation of Castle episode 7.03 "Clear and Present Danger"... with a little Supernatural twist.
Ever since I saw the episode of Castle where the guy “makes a deal with the devil,” I knew this had to happen.
It just took me a while to do it.