What significant event marked the end of the Second Triumvirate?

The end of the Second Triumvirate was significantly marked by the defeat of Marc Antony by Octavian at the Battle of Actium in 31 BCE. This battle was a decisive naval engagement in which Octavian's forces defeated those of Antony and his ally Cleopatra, leading to their eventual suicides in 30 BCE and marking the collapse of the Triumvirate. Following this victory, Octavian consolidated power and later became Augustus, the first Roman Emperor, fundamentally transforming the Roman political landscape and effectively ending the Republican system that had previously characterized Rome.

The other events listed, such as the assassination of Julius Caesar, occurred earlier and were part of the events that led to the formation of the Triumvirate rather than its conclusion. The establishment of the Roman Empire is a broader outcome of Octavian's victory but is not the event that immediately signaled the end of the Triumvirate. Meanwhile, the formation of the First Triumvirate is unrelated to the Second Triumvirate and highlights a different period within Roman history.