Everything has a price. The smallest of actions, both good and bad, can place many into the grave. The roots run deep, after all, in any and all Equestrias.
Twilight Sparkle didn't consider what might happen when she used an "Object Possession" spell to check on her student. Now she has to live with the consequences. Forever.
Cadance and Twilight have discovered something about the Alicorn Amulet. Whenever someone rejects its promises of power, it weakens the Amulet's magic. They summon the help of volunteers from across Equestria, in the hope of destroying it for good.