Deep under the resplendent city of Canterlot lies a web of vaults, containing many secrets hidden from the common folk. One of such undercrofts shelters an extraordinary artifact – the crimson horn of King Sombra, the uneasy spirit of the corrupted unicorn still within it.

After months of careful consideration, Princess Celestia ventures down into the delves underneath to confront the fallen king, determined to aid him and bring him serenity at long last.