Complex sentence: When Shaddam starts to act without Fenring's counsel, that is when he starts to make grievous mishaps: using atomic weapons, a biological plague, and threatening to destroy Arrakis (' Dune') being three of those serious mistakes.
Split into simpler sentences: When Shaddam starts to act without Fenring's counsel, that is when he starts to make grievous mishaps. Using atomics, a biological plague and threatening to destroy Dune being three of those serious mistakes.