I like to ask. Did the Royal Sister use the Elements of Harmony on King Sombra, or there own powers?
I want to write a story about why the Tree of Harmony is in Everfree when it looks like it belongs in the crystal ponies lands.

The order of events for the story to work.
1)Discord transplant the Tree of Harmony to Everfree
2) King Sombra enslaves the crystal ponies.
3) Princess Celestia and Princess Luna go to the Crystal Empire.
4) Battle King Sombra, but fear they lost the Tree of Harmony.
5) Learn of the tree being transplanted.
6) Battle a King Everfree, who is a forest version of King Sombra.
7) Win the Tree of Harmony.
8) Turn Discord into stone.

Does this look a good story line?