Ponykind has many legends.

They tell of terrifying monsters, fearsome beats, and crippling illnesses. The Mare in the Moon, Daring Do and Ahuizotl... They are all based on realty, though some are more fanciful than others. Ponies everywhere enjoying re-telling them to friends over a mug of cider, or to young fillies too restless to fall asleep. Still, one story is rather taboo in Ponyville. It tells of a sickness that cannot be cured. The reason only brave colts whisper it to each other in dark alleyways is because it is too close to home. The sickness has visited Ponyville before.

The Apple Family is all-too-familiar with this sickness, known too many as the dark plague. Several years ago, many ponies died in a deadly epidemic. The Ponyville chapter of the Apples was very nearly disbanded by the fearsome killer, and it still managed to orphan Big Macintosh and Applejack. Now, old nightmares are rising from the black pits of the past. Applejack, the bearer of Honesty and most dependable pony around, has fallen ill. Though few realize it, she is in very grave danger. No pony magic can cure the dark plague.

Desperate to keep her family together, Apple Bloom and her friends have vanished into the Everfree Forest, to seek the aid of a herbalist. Perhaps the skills of a zebra can save her sister. What they do not know is that there is but one race that can stop the Plague. A race that is only spoken of in legend and myth. A race that has not been seen since Princess Celestia was but a filly...