Complex sentence: Using a mysterious Gallifreyan crystal, Ronan was able to make all the Cybermen follow Braxiatel's orders; as the crystal was slowly killing Ronan, it was arranged for Jason to arrive on Cantus and became the new Controller.
Split into simpler sentences: Using a mysterious Gallifreyan crystal, Ronan was able to make all the Cybermen follow Braxiatel's orders. As the crystal was slowly killing Ronan, it was arranged for Jason to arrive on Cantus and he became the new Controller.