Complex sentence: The six ponies enter the castle where they find five of the six Elements, a set of stone spheres, the sixth of which Twilight says will only appear via a "spark" once the other five are present.
Split into simpler sentences: The six ponies reach the castle where they find five of the six Elements, a set of stone spheres. Twilight tells the others that the sixth element will only appear via a "spark" once the other five are present.