An eternity is a far too long to keep a secret.

When Princess Celestia is warned of a threat to Equestria, one with the potential to tear the world apart, and ultimately steal her own sister from her, she vows to devote her life to finding a way to avert this disaster and save all those she loves from their fate.

Deep down, however, she knows that she can never truly achieve this, and that protecting her sister completely would require sacrificing everything else.

The Codex, compiled by Celestia herself, is the only full account of the events, spread over millennia, which led to her greatest loss, and her efforts to redeem her sister and restore the world to its natural balance.