Complex sentence: Krang spent the next two years in Dimension X until he was contacted by Dregg, who arranged for him and Shredder to come back to Earth to help him fight the Turtles.
Split into simpler sentences: Krang spent the next two years in Dimension X, until he was contacted by Dregg. Dregg arranged for him and Shredder to come back to Earth, to help him fight the Turtles.