Complex sentence: He was the third son of Roger of Montgomery, 1st Earl of Shrewsbury and Mabel of Bellême, and came by the appellation "the Poitevin" due to his marriage to an heiress from Poitou (see below).
Split into simpler sentences: He was the third son of Roger of Montgomery, 1st Earl of Shrewsbury and Mabel of Bellême. The appellation "the Poitevin" was for his marriage to an heiress from Poitou (see below).