Complex sentence: They assassinated the two leading Zygons with the intention of exposing the 20 million Zygons living on Earth and ending the peace treaty, their member Bonnie assuming the form of the Doctor's companion Clara in a plan to eliminate UNIT and the Doctor.
Split into simpler sentences: They assassinated the two leading Zygons with the intention of exposing the 20 million Zygons living on Earth and ending the peace treaty. Their Bonnie assumed the form of the Doctor's companion Clara in a plan to eliminate UNIT and the Doctor.