Historically, the later period of the Western Roman Empire was heavily shaped by the migration of barbarian Germanic tribes into their former lands, paving the way for medieval states. Imperator: Rome, Paradox’s upcoming grand strategy game about the classical world, starts several centuries earlier than that in a time when Rome was just beginning its rise to power. But when I heard barbarian migrations had been implemented as a launch feature, I didn’t let the date on the calendar stop me from kicking the party off early.
Imperator is based around population units called pops, a concept that may be familiar to players of other Paradox games like Stellaris and Victoria II. These are the people that live in all of the thousands of cities on the map. They can be encouraged to move, they can decline and die, or they can multiply as history marches on. Germanic barbarian tribes have the ability to uproot their pops and turn them into mobile armies, leaving their former homes uninhabited and allowing them to seek new ones.
Source: IGN.com Imperator: Rome Has Deadly Forests and Emergency Dictator Powers