The Huns and the Great Migrations
Horse archers from the Eurasian steppe arrived on Europe's eastern edge and pushed the Goths, Vandals, and others west into Roman territory — not as invaders at first but as refugees asking for land. Rome admitted them, mistreated them, and fought them. Under Attila the Huns extorted enormous tribute from Constantinople and raided Gaul and Italy before his empire dissolved within years of his death. The Huns themselves left almost nothing behind; what they left was everyone they displaced, now living inside the empire.
Large movements of peoples — pushed or pulled — reshape the societies they leave and the ones they enter.