My Little Pony XCVII: Cheerilee's Lonely Hearts Club

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Spoiler: EQG Overpowered short

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Interesting limitation on the geodes power. The more the magic is used the stronger it gets. Eventually becoming uncontrollable and manifesting at random. At which point the girls need to stop using magic at all to allow it to dissipate. Against a crisis they need to use magic against but can't overcome quickly the girls are in danger of becoming a bigger threat themselves than whatever it is they're trying to stop.