Shadow of a Sprout

(Originally published on FiMFiction, based on a fan comic and tagged AU as a result.) Sprout runs away from Maretime Bay and makes an unusual "friend". Said friend convinces Sprout to turn on his other friends. When things look bleak for Sunny and the gang, some unexpected faces show up to help. Can this team of heroes save Sprout from The Pony of Shadows' grasp?