Complex sentence: The team is formed by Tess Mercer to track down Doomsday.Tess Mercer took over at LuthorCorp, she shut down the Black Creek facility and many metahumans escaped.
Split into simpler sentences: The team is formed by Tess Mercer to track down Doomsday. After Tess Mercer took over at LuthorCorp, she shut down the Black Creek facility, and many metahumans escaped.