Final answer:
Following the historical patterns and events leading up to his reign, Caligula was likely assassinated by the Prefect of the Praetorian Guard due to his tyrannical rule, with Claudius taking over as emperor. Similarly, Darius of the Persian Empire would consolidate his power post-rebellion. Political instability was a common motif during these times, often leading to assassination and succession of new leaders.
Step-by-step explanation:
The event that most likely occurred next during his reign, given the historical context provided, would be the assassination of Caligula and the subsequent ascension of his uncle Claudius as the new emperor. The description of Caligula's transition from a noble ruler to a tyrant due to an illness or possible poisoning outlines a degrading situation which culminated in his assassination by the prefect of the Praetorian Guard. This action was taken as a direct consequence of the instability and fear that Caligula's behavior had instilled.
Another potential event that may have followed the discussed circumstances would involve Darius's consolidation of power as the Emperor of the Persian Empire. This power consolidation included quelling rebellions and establishing legitimacy through extensive propaganda, such as the Behistun Inscription. Darius also secured his lineage through strategic marriages.
For a broader context, the Roman Empire during this period faced significant challenges due to political instability and frequent changes in leadership, which points towards the pattern of a repeating cycle of assassination and the rise of new leaders, as was the case following the death of Nero and the events leading up to the 'Year of the Four Emperors'.