Complex sentence: Before the ride starts Wendy Christensen had a premonition that the roller coaster would crash; both Erin and Ian fall to their deaths after the ride gets stuck on a vertical loop.
Split into simpler sentences: Before the ride starts Wendy Christensen had a premonition that the roller coaster would crash. In Wendy's premonition, both Erin and Ian fall to their deaths after the ride gets stuck on a vertical loop.