Luna: What are we going to do if Malefor returns and strikes again?
Celestia: We have to use the Tree of Harmony to seal him in and this time, he will stay imprisoned.
Luna: We would have the bearers give up their Elements?
Celestia: That's not entirely true. *show images of Spike morphing into Harmonic Spike* Twilight and her friends are the bearers of the Elements of Harmony.....but Spike contains the same abilities as the six, but he has the power to morph into Harmonic Spike. The only way to set off the Tree of Harmony is to have Spike morph in the vicinity of the tree.
Luna: Only if Malefor strikes at Equestria again.
Celestia: Only if he strikes back.