Complex sentence: Dawn agreed to use her memories to restore Earth while Silver Surfer left to restore the rest of the universe, but Silver Surfer unmade Galactus while Dawn unknowingly created another version of Norrin.
Split into simpler sentences: Dawn agreed to use her memories to restore Earth while Silver Surfer left to restore the rest of the universe. But Silver Surfer unmade Galactus while Dawn unknowingly created another version of Norrin.