When Twilight first formed the Land Heart group to explore the deepest reaches of the land beneath the surface, she only planned to make discoveries of archaeological and possibly historic interest. She certainly didn't expect to unleash an ancient evil older than pony kind.
...she probably should have, though.
Now, her friends and her exploration group are Equestria's first line of defense against these ancient evils that have crawled out of the depths to claim the surface.

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Crossover with Inhumanoids