If Ponyville's new repair-pony gets his way, the Mane Six won't even know he exists - and nopony in Ponyville will miss him for long once he's returned to Earth.
A long forgotten valley is appropriated as the sanctuary for a new changeling hive. Does this involve the long dead sunstone? Celestia thinks it might and Cadence has her own reason to tag along with the Elements, vengeance!