1000 years of unrest and unhappiness under the reign of Discord.
That's all there is to describe what happened in that time period in the Canterlot archives.
However, Discord himself is just the tip of the iceberg.

Before the being known as Discord came to be, a cold war between Ponykind and Diamond dogs took place after they teamed up to defeat a common enemy : the Changelings.

The Holy Royal family of ponykind, settled in the City of Crystals hidden up in the north, guides and protects the people of their kingdom all the way down to the Everfree Forest.
The Strong Emperor of the Diamond Dogs rules upon the southern portion of the continent up to that same forest, which has remained ,through all conflicts, a neutral territory.

Other than their brutality and mercilessness in combat, not much is known about the Diamond dogs. They turned down all offers of discussing a peace treaty with the Holy King of Ponykind. The cooperation to beat the changeling threat happened purely by coincidence, according to the military officials. After that, an unofficial time of peace took place, allowing both civilizations to replenish themselves from the past battles, but the threat of a new series of bloody battles is still very real.

Thought Flare, a young colt unicorn living in Canterlot at that time period, grows up to see the society he lives in, in a much different light than anyone else, and he doesn't like what he sees. As his name indicates, he gets big ideas, but they tend to be of the flaming type, metaphorically, but literally as well. Because of this, Thought Flare will witness and accomplish things that will bring the books to have more than "1000 years" worth of blank pages.