Days after the Elements' battle with Tirek, Twilight learns that not everything went perfectly with his banishment to Tartarus – and that something is very wrong with her.

Now Twilight is stuck with a parasite of magic and hellfire inside of her – one that's slowly hurting her friends and everypony she knows, namely Rainbow Dash – and she's determined to fix everything and give back everything she's inadvertently taken.

No matter what it takes.