Fortify Castle was once a normal pony just trying to get by. This all changes when his family is killed by a mysterious mutant and he now seeks to protect the rest of Equestria by any means necessary. The path he now leads will take him through a whole new and darker world then the one he once knew and he plans to bring punishment to anypony or mutant that gets in his way.

A side story of KoolerKids My Little Avenger
This story can be read without reading his though it would greatly diminish the value to do so.