Dr.Weil did something to Ragnarok, so when it exploded Zero would no longer return... But did Zero really die?

When Lucy died, a dark spirit revived her. The spirit was called, the "Nightmare"

Twilight Sparkle creates a drug in her basement on accident, while a seemingly harmless gas is in the air. She brings her best friends to see it, but she doesn't realize she has infected her friends...