Complex sentence: The new Earl of Douglas, and Sir David Lindsay mustered their men joined forces with the French knights and led a raid into England where the ravaged lands belonging to the Percy Earl of Northumberland, and the Mowbray Earl of Nottingham.
Split into simpler sentences: The new Earl of Douglas, and Sir David Lindsay mustered their men and joined forces with the French knights. They then led a raid into England where they ravaged lands belonging to the Percy Earl of Northumberland, and the Mowbray Earl of Nottingham.