Joined Forces (Supernatural/Arrow crossover)...
Dean Winchester and his brother Sam are on a case that seems very suspicious. They think that the Starling City vigilante is a demon. The two brothers drive up to Washington, guns in hand.
Oliver Queen, dressed as Arrow, has been stopping bank robberies in lack of anything else to do. Soon, Oliver notices someone hot on his trail.
Could the two of them join forces to take down a threat to them both?
(If you only watch one of the shows, that's cool, you'll still get it... Hell, if you don't watch either, you'd still probably get it)