Complex sentence: The Visigothic kingdom was already severely reduced in power and area at the time he succeeded Achila II, and his dominions probably didn't extend beyond Septimania and present-day Catalonia, due to the Arab conquests of the previous three years.
Split into simpler sentences: The Visigothic kingdom was already severely reduced in power and area. At the time of succeeding Achila II on the throne, his dominions probably didn't extend beyond Septimania and present-day Catalonia, due to the Arab advance of the previous three years.