Complex sentence: This victory proved to be short-lived as Paul tells us of 2,000 Bavarians, who were slain to a man in 595 when invading the lands of the Slavs to help the Kakan (chief of the Avars).
Split into simpler sentences: This victory proved to be short-lived as Paul tells us of 2,000 Bavarians, who were slain to the last man in 595. They had invaded the lands of the Slavs, who received help from the Kaghan (chief of the Avars).