(d. 942) Son of Rollo, William count of Rouen (r. c.928-42), was the second ruler of Normandy. William continued the expansionist policy of his father and was granted the Contentin peninsula in 933 and attempted to establish his protection over Brittany. In 935 William invaded Flanders but was defeated at Therouanne. William also consolidated his authority over the Scandinavian settlers in Normandy, putting down a rebellion led by a Viking called Rioul in 933-4. In 939 he allied himself with Hugh the Great in a rebellion against King Louis IV. The war was ended through the mediation of the pope, and Louis confirmed William's investiture of Normandy in 940. William's murder, on the orders of Arnulf I, count of Flanders, during a meeting on an island in the River Somme in 942, threw Normandy into chaos. He was succeeded by his illegitimate son Richard I (Richard the Fearless).
Notes from Encyclopaedia of the Viking Age by John Haywood. Thames & Hudson 2000. p 208.