The Second Rebellion occurs towards the end of your campaign. Namely, it occurs when you reach 85 settlements.
This is a simulation of a combination of the Republican Civil War of Julius Caesar, the Civil War of Octavian, as well as the tumultuous civil war of the Year of the Four Emperors. Entire provinces will be lost, large swathes of land will turn themselves to the Roman Rebels Faction, and your armies will be near decimated if the rebellion is big enough. This is the defining moment of any Roman player's mettle. Do you retreat towards a base of operation and slowly rebuild? Will you trade space and lives for time? Will you mount an all out assault against Rome and the feared Praetorian legions in the hopes of regaining your homeland? Will you ultimately regain control of your empire or die trying? Conversely, if the rebellion allows you Italy, will you crush the rebels beneath your boots? Or will a few ambitious men overthrow your hold of power and sweep away all that you have worked for? And all while fighting the rebellion, the external enemies of Rome are eyeing her weakness and are all preparing to take advantage of a rich divided empire...