Complex sentence: The last known pure-blood descendants of Hogwarts founder Salazar Slytherin, the Gaunt family had an obsession with blood purity which spurred them to marry their own cousins through the generations, resulting in mental instability.
Split into simpler sentences: The last known pure-blood descendants of Hogwarts founder Salazar Slytherin, the Gaunt family had an obsession with blood purity. This led them to marry their own cousins through the generations, resulting in mental instability.