While journeying out into the Royal Labyrinth, Celestia happens upon a rather sorry-looking draconequus by the name of Eclipse Tumult. While he's skittish at first, she eventually earns his trust. With his help, Princess Celestia hears Discord and Eclipse's story from a new perspective: one leading back to times long before she even existed...

Eclipse was Discord's "Brother", in a manner of speaking. The two had grown up together, moved from place to place together, and been shunned and hated together. Eclipse never left his brother, however... Not until he no longer had one.

May become romantic later on, but we'll see. If so, I'll add that tag.