Taken from here

“Twilight Sparkle turned into a monster from her necklace because she stole all the power. She took Rainbow Dash’s power and Rarity’s and Pinkie Pie’s power. She made [the necklace]. She needed some help. She didn’t know it would turn her into a monster. But Sunset Shimmer stopped her. And the bad girls from the different school, Crystal Prep, just changed their minds.”

I AM SO ECSTATIC THEY TURN NICE!!