Twilight Sparkle worries that she might one day grow bitter and spiteful, and turn into somepony akin to Nightmare Moon. To combat this, Twilight prepares a spell that will let her go inside her mind and battle her negative personality traits while they're still weak and unformed. It doesn't end well.
Rarity and Spike eventually reconstruct the notes that Twilight used to create her spell, and decide to go inside Twilight's mind to wake her from her magically-induced coma.
It also doesn't end well.
But a bad ending isn't going to stop Rarity and Spike from trying to rescue Twilight anyway.
Chapters (3)
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How To Be You - Lewis And The Strange Magics
Well, this certainly does an excellent job of presenting a clear and present danger to all the characters involved. I'd expect Spike to be a little more broken-up about what has happened to Twilight, but then again that's evened out by the absolutely wonderful interplay between Spike and Rarity. About 500 extra points for using the word "susurrus" as well. Good job, and I look forward to seeing where this goes.
I feel like I've seen these words before, can't remember where.
6561810 You're probably thinking of this: encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcQcpxZLzBOyPQSEeJ9NV7V-8zvtiOk5vi6t-LA7k4GfHsObxKl8YLh87g