Complex sentence: Alavivus (flourished in 4th century AD) was a Gothic chieftain who together with Fritigern led the migration of the Thervingi from Dacia across the Danube into the Roman Empire in the late 4th century AD.
Split into simpler sentences: Alavivus (flourished in 4th century AD) was a Gothic co-king of a group of Thervingi together with Fritigern. Along with the latter he led the migration of the Thervingi from Dacia across the Danube into the Roman Empire in the late 4th century AD.