After being fatally injured by an unknown assailant. Asriel is transported to the magical land of Equestria. He will the magic of friendship and how it could help him return home with a soul that can save him from the fate of being a flower again.
The barrier preventing monsters from returning to the surface was broken. Yet a fail-safe was placed to ensure that the humans would never see them again. New creatures appear in a cave in the Everfree, and nopony is prepared for their new visitors.
Brothers to the end. Where one went, the other followed. Unfortunately for them, that meant going to a land they know nothing about. Fortunately for you, their adventures in said land have been leaked out to the public. How scandalous!