The Five Souls
Many ponies do not know about the legend of the Soul Stealer. The Soul Stealer is an ancient creature that was banished for all eternity by Princess Luna. However, it is said that if one were to make a sacrifice of five souls, the Soul Stealer would be released from their prison. Although it has been attempted, only true corrupted souls could be used for sacrifice. If a soul isn't corrupted they will be freed.
It was at this point that Twilight stopped reading. She understood everything. Pinkie Pie, Rainbow Dash, and Fluttershy were not corrupted.
Twilight soon found the page she was looking for and jammed the key into the keyhole. A bright, warm light filled the room, and a magic portal appeared.
“Come, Spike. Let's go home.” the princess said, grabbing her young assistant.