tl;dr how the current celestia rose to power , found the elements of harmony and defeated discord.

(This takes place over a thousand years back from current mlp canon)
The current Celestia is dying as does happen every couple thousand years. Equestria is in a state of civil unrest and war in eminent, thus Celestia knows she must pass on her title and begins a program for gifted Alicorns to help decide.

Dawn Everlight is an Alicorn who was just accepted into Celestia's program which is designated to chose the new ruler of Equestria as well as her two best friends Luna and Princess Mi Amoure Melody. Dawn isn't the most studious but she has great power and diplomatic skill which makes her desirable in the empire's current state.

She meets a colt named Vanishing Act who she falls deeply for. They plan to spend their lives together until Dawn is chosen to be the next Celestia and must choose between love and duty.

In an effort to stay with her forever and let her be Celestia, Vanishing dabbles in chaos magic attempting to tame its forbidden powers in order to become immortal. After a failed spell, he is transformed into a Draconequus which has not changed Dawn's love. He promises her he will stop however the addiction is too strong and the withdrawl drives him mad. He is banished by Celestia out the premonition of war. Separated from Dawn, and left imprisoned in his thoughts he becomes a monster and sieges war over Equestria with his army of disorder.

Now Dawn must rise up to fight the monster that has plagued her true love as well as save the kingdom which is destined to be hers.