On Equestria, there is no place quite so unknown as deep under the earth. It is known that caverns go down. And down, and down, and down, breaking even the hardiest of explorer's will to keep descending. Nopony truly knows where those caverns go, what's inside of them, of really, why they're even there at all. they just know their old, and they're right:

Those holes are old as the dawn itself. And they're not empty. Far, far down under the earth, something's down there. Many something's; ancient somethings. Once, in a long forgotten age, they hadn't always been down there.

And now something's come up. And a wish to take a life into one's own hooves will upset actually everything. Everything. This normally innocent act might even bring a cataclysm unthought of to Equestria, in fact.

But it just might cause an ancient grudge to come to an end, break a prophecy, and reunite that which has for so long sat asunder than neither part recalls being of the same whole.