Complex sentence: Qyburn reveals to Cersei a ballista designed to be effective against dragons; it pierces the skull of Balerion, Aegon the Conqueror's steed.
Split into simpler sentences: Qyburn reveals to Cersei a ballista designed to be effective against dragons. Demonstrating it, it pierces the skull of Balerion, Aegon the Conqueror's steed.