Complex sentence: The second event is Superman's creation of an alternate reality in the Phantom Zone, which he meant to use as a refuge for the people of Earth in case a tragedy such as the one that befell Krypton were to occur.
Split into simpler sentences: The second event is Superman's creation of an alternate reality, which he placed in the Phantom Zone. He meant to use this reality as a refuge for the people of Earth in case a tragedy such as the one that befell Krypton were to occur.