Complex sentence: The seventh Duke had no sons and was succeeded by his third cousin, Ferdinand Cavendish-Bentinck, 8th Duke of Portland, a great-grandson of Major-General Lord Frederick Cavendish-Bentinck, fourth son of the third Duke.
Split into simpler sentences: The Seventh Duke had no sons and was succeeded by his third cousin, Ferdinand Cavendish-Bentinck, 8th Duke of Portland. Ferdinand was the great-grandson of Major-General Lord Frederick Cavendish-Bentinck, fourth son of the third Duke.