After Rainbow Dash goes one step beyond a sonic rainboom, the magic emitted from such a feat awakens an ancient being, older than time and more powerful than Celestia.
Spike knew his services were no longer required after all those guards and servants and... He moved in. Twenty five years later, Twilight's negligence comes back to bite her.
After a strong emotional trip, Luna accidentally raises the moon for the first time. This is how the High Council of Equestria knows it is time for the prophecies to come true and to hand over the rule of Equestria.