Complex sentence: On leaving Exxilon, the Daleks will fire a plague missile onto the planet; destroying all life and making future landings impossible, so the Daleks will have the only source of Parrinium.
Split into simpler sentences: On leaving Exxilon, the Daleks will fire a plague missile onto the planet, destroying all life and making future landings impossible. By doing so, they will have the only source of Parrinium.