Sins of Harmony: Volume 1, Balance of Power

Something brews in the East hemisphere, as Equestria encounters threats beyond Nightmare Moon 5 years ago, or Tirek 3 years after. The magic of friendship can't always prevail, as the will to survive overcomes all. But Celestia knows this, as one she regrets crossing returns with intents unknown, unbeaten by the Elements with his unyielding will.