Complex sentence: In September 2007, Megadeth returned to the United States as the headline act on their "Tour Of Duty" tour, which included the Pacific rim and the third installment of Gigantour, which was launched in Australia in November 2007.
Split into simpler sentences: In September 2007, Megadeth returned to the United States as the headline act on their "Tour Of Duty" tour. The third installment of Gigantour was launched in Australia in November 2007.