Earl William Mortaigne CORNWALL
Other names for William were CORNWALL Earl and MORTAIN.
Earl of CORNWALL.
The Political History of England, Vol II, George Burton Adams Longmans Green and Co, 1905, Ch I, p138:
 "In this same year, following out what seems to have been the deliberate purpose of Henry to crush the great Norman houses, another of the most powerful barons of England was sent over to Normandy, to furnish in the end a strong reinforcement to Robert of Belleme, a man of the same stamp as himself, namely William of Mortain, Earl of Cornwall, the king's own cousin. At the time of Henry's troubles with his brother Robert, William had demanded the inheritance of their uncle Odo, the earldom of Kent. The king had delayed his answer until the danger was over, had then refused the request, and shortly after had begun to attack the earl by suits at law. This drove him to Normandy and into the party of the king's open enemies. On Henry's departure, Robert with the help of William began again his ravaging of the land of his enemies, with all the former horrors of fire and slaughter. The peasants suffered with the rest, and many ofthem fled the country with their wives and children."
p144:  "The battle upon which Henry [I] embarked in August ended by the close of September in a success greater than he could have anticipated. He first attacked the castle of Tinchebrai, belonging to William of Mortain, and left a fortified post there to hold it in check. As soon as the king had retired, William came to the relief of his castle, reprovisioned it, and shut up the king's men in their defences. Then Henry advanced in turn with his own forces and his allies, and began a regular siege of the castle...
"The battle was fought on September 28, and it was fiercely fought, the hardest fight and with the largest forces of any in which Normans orEnglishmen had been engaged for forty years. The main body of both armies fought on foot. The Count of Mortain, in command of Robert's first division, charged Henry's front, but was met with a resistance which he could not overcome. In the midst of this struggle Robert's flank was charged by Henry's mounted allies, under Count Elias of Maine, and his position was cut in two. Robert of Belleme, who commanded the rear division, seeing the battle going against the duke, took to flight and left the rest of the army to its fate. This was apparently to surrender in a body. Henry reports the number of common soldiers whom he had taken as ten thousand, too large a figure, no doubt, but implying the capture of Robert's whole force.His prisoners of name comprised all the leaders of his brother's side except Robert of Belleme, including the duke himself, Edgar the English atheling, who was soon released, and William of Mortain. The victory at once made Henry master of Normandy. There could be no further question of this..."