Complex sentence: Omniverse, the fourth series, is more similar to the first series than the second and third, which doesn't have the dark tone, complex plots, and more mature characters of the second and third series.
Split into simpler sentences: Omniverse, the fourth series, is more similar to the first series than the second and third. Like the first series it doesn't have the dark tone, complex plots, and more mature characters of the second and third series.