Complex sentence: During the 5th and 6th centuries, gold and silver become more and more common, but this gold arrives during the pillaging of the Roman Empire by Germanic tribes, and from which many Scandinavians returned with gold and silver.
Split into simpler sentences: During the 5th century and 6th century, gold and silver become more and more common. This time saw the pillaging of the Roman Empire by Germanic tribes, and from which many Scandinavians returned with gold and silver.