Complex sentence: Scylla is first mentioned in the season 4's premiere episode Prison Break and is said to be The Company's "blackbook" that's needed to help that series convicts to bring down The Company and help them escape a jail sentence.
Split into simpler sentences: Scylla is first mentioned in the season 4's premiere episode Prison Break. According to the plot Scylla is introduced to the audience as The Company's "blackbook" that's needed to help that series convicts to bring down The Company and help them escape a jail sentence.