Complex sentence: After succeeding his father as 7th Duke of Somerset he was created Earl of Northumberland in 1749, the Earldom of Northumberland having become extinct with the death of his maternal grandfather in 1670.
Split into simpler sentences: After succeeding his father as 7th Duke of Somerset he was created Earl of Northumberland in 1749. The Earldom of Northumberland having become extinct with the passing of his maternal grandfather in 1670.