After Starlight's reformation, she still feels guilty for what shes done.
The Tantabus returns and persuades Starlight to free her guilt on her. Starlight agrees, and turns into Nightmare Equality. She now not only wants to make ponies equal, but all creatures. She is now unstoppable. An unlikely group of friends (or not so much friends) team up to stop her with the Magic of Friendship. But the very reason that pushed her over the edge may be just the one to stop Nightmare Equality, and save Starlight Glimmer