Complex sentence: The three rush to get the bow and arrow Roland used to slay the dragon but Flagg catches up with them and is about to kill them when Thomas reveals himself and the fact that he watched Flagg poison Roland.
Split into simpler sentences: The three rush to get the bow and arrow Roland used to slay the dragon but Flagg catches up with them. The wizard is about to kill them when Thomas reveals himself and tells Flagg how he watched him poison Roland.