Complex sentence: Arechis was descended from a Germanic group, called the Lombards (or Longobards), who had invaded the Italian peninsula in the late sixth century and established a Lombard kingdom in northern Italy with its capital at Pavia.
Split into simpler sentences: Arechis was descended from a Germanic group, called the Lombards (or Longobards), who had invaded the Italian peninsula in the late sixth century. At that time, the invaders had established a Lombard kingdom in northern Italy with its capital at Pavia.