There is an ancient story that tells of a powerful ruler who went mad and destroyed an entire country. Two shipwrecked explorers meet that very land and the ruler who caused it all. Now they have the opportunity to listen to their side of the story.
Celestia sits upon her throne forever still and weeping. She had failed to protect her subjects, she had failed to save the Crystal Empire, she had failed to save her sister. So when a new aggressive threat comes to Equestria declaring war, she aims to makeup for all of her failures but her desperation only serves to create one more which finally brings her to the brink. For Luna, is was failing to be her subjects standards that drove her mad. For Celestia, it was not living up to them. In an attempt to create a better place she became a tyrant and in the end a tyrant is what she became.