Complex sentence: Beaufort is a picturesque figure in French history of the 17th century, was the second son of César de Vendôme, an illegitimate son of King Henry IV of France by his mistress, Gabrielle d'Estrees.
Split into simpler sentences: Beaufort is a picturesque figure in French history of the 17th century. He was the second son of César de Vendôme, an illegitimate son of King Henry IV of France by his mistress, Gabrielle d'Estrees.