Why Me? (Book 3 Arrow/SPN Crossover Series)
11 parts Complete (Written in 2014) Lucifer has trapped Sam in his own mind. Castiel taps Oliver to help Dean face his feelings about his family and his brother. If Oliver can't help Dean, Sam may not be able to continue to fight Lucifer and stop the Apocalypse. (Read A Pointed Journey and Back from the Brink first)
(Two things about this story. 1. The opening scene is still my favorite scene I have written to date 2. The chapter with Remiel ended up breaking a several year long writer's block I had for what ended up becoming Testament of An Archangel. His scene is pretty much the outline for that book, so consider this a bit of a spoiler alert for Testament. :) )