Have you lost a dear one long before their time? Accidents, monsters, foreign attack... they can all leave you with a hole in your heart. We can fill it, and give your loved one another chance.

Twilight's little spell does not remain as secret as she would have liked it. Now another, less scrupulous, soul has it and means to make good use of it. There are aliens that are not only willing, but eager to step into the hooves of a pony. Matched up with a pony in need, and everyone wins. Right?

Do you know me, the author? I don't tend to wallow in darkness, but the theme of the story, the central conceit, is pretty dark, so there's the tag.