Discord and Celestia, as well as Luna. That these three share a history is no doubt, even though an official one has eluded me.
This is my tackle on the subject, how such a powerful creature as Discord came to be in the first place, his relationship with his beloved princess and her younger sister, and the creation of pony-land as we know it today.
Contains clop-material, fan-written backgrounds to what the show never told, and a possible future for some well-loved characters of the mane six.
Also people, do not vote it down because "oh my god clop fic even though I was warned about it!"
If you do not want to see it, read it first.