Complex sentence: It was under Seigbert's reign that the Mayor of the Palace began to play the most important role in the political life of Austrasia, and he has been described as the first "roi fainéant"--do-nothing king--of the Merovingian dynasty.
Split into simpler sentences: It was under his reign that the Mayor of the Palace began to play the most important role in the political life of Austrasia. He has been described as the first "roi fainéant"--do-nothing king--of the Merovingian dynasty.