During Prince Sombra's venture into the Crystal Mountains, an ancient evil awakened that began corrupting him from the inside out. After his violent outburst during the jousting tournament of the Crystal Fair, Princess Sterling realizes her vision wasn't just a dream, but a prophesy.
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This story is based off of a head-canon I have about Princess Sterling and Princess Gold Lily, the newest Princesses introduced in the "Rainbow Power" line of MLP brushables. It is still unknown if they will actually appear in season 5 or not, but I assume they will turn out like how Princess Skyla did.