It is unknown at this time, as it is meant to be a mystery. This will likely be the story arc for season 4, so the contents of the box will probably not be revealed until near the end of the season.
The My Little Pony Friendship is Magic Rainbow Power toys are connected to the box on the Root of the Tree of Harmony show.
Most likely
No, that box transformed into castle
My Little Pony: Friendship is Magic
yes
My Little Pony: Friendship is Magic premiered on October 10, 2010.
S2 E1 "Return of Harmony"
The Crystal Empire is the Rainbow Kingdom in your Little Pony Friendship is Magic.
My Little Pony: Friendship is Magic premiered on October 10, 2010.
My Little Pony Friendship Is Magic - 2010 Friendship Is Magic Part 2 Elements of Harmony 1-2 was released on: USA: 22 October 2010 Italy: 31 August 2011 Poland: 15 October 2011 Ukraine: 28 January 2013 Japan: 9 April 2013
My Little Pony Friendship Is Magic - 2010 The Return of Harmony Part 2 2-2 was released on: USA: 24 September 2011 Ukraine: 1 July 2013 Japan: 8 October 2013
My Little Pony Friendship Is Magic - 2010 The Return of Harmony Part 1 2-1 was released on: USA: 17 September 2011 Ukraine: 1 July 2013 Japan: 1 October 2013