Alternative Season 10 of Supernatural. Cas' perspective as to why this new problem is far worse than anything Team Free Will has ever had to deal with before. Includes brotherly comfort between Cas and Sam, and a slow burn of Cas and Dean.
An Apocalypse of the undead has been going on for a while, Sam Dean and Castiel have survived this long, but lately they have been having to go further and further away from the bunker to look for resources