Complex sentence: The princesses inform the ponies that the mirror leads to a different world, and task Twilight with retrieving her crown on the other side before the portal closes again for thirty moons, or else the Elements of Harmony borne by her friends will no longer protect Equestria.
Split into simpler sentences: The princesses inform the ponies that the mirror leads to a different world. Celestia tasks Twilight with entering this world to retrieve her crown before the portal closes again for thirty moons, or else the Elements of Harmony borne by her friends will no longer protect Equestria.