A while after the events of "Rarity takes Manehattan," the other five have also gotten their keys (I know you know that spool of rainbow thread is a key) and opened the box. There didn't seem to be anything inside. They can't figure out what's up with that until a week later, when they start developing superpowers. That's when things get kind of strange.
Written for the EqD Writer's Training Ground #002. Comments are encouraged, because I want to know what people think and I don't expect many people to read this and thus be available to provide thoughts.