Following near the end of season 1's battle against Nightmare Moon, Princess Celestia tries a different approach to defeat her transformed sister if the Elements Of Harmony wouldn't work. Thankfully having a smart, bookworm like her faithful student, Twilight Sparkle, is very hardy to finding spells old or new that could defeat the mare in the moon, even dark magic.
However, messing around with dark magic that nopony understands could lead to effects that weren't the desired result...