Discord is the antagonist featured in The Return of Harmony Part 1 and Part 2. In the episode, Princess Celestia explains that before their rule over Equestria, she and Princess Luna had imprisoned him in stone using the Elements of Harmony, but now that the sisters are no longer connected to the elements, the spell has been broken. Discord breaks free of the spell and sends Twilight Sparkle and her friends in a quest after the missing Elements of Harmony. The ponies venture off to retrieve the Elements of Harmony in order to stop him.
And he became as a Anti Hero in Season 3 Episode 10.