After Rollo, its rulers included Robert Curthose and Robert the Devil.  
This region is home to the island of Tatihou, which lies to the east of the Cotentin Peninsula. Another portion of it is the Pays de Caux, where the Cauchois dialect is spoken.  During the Middle Ages it was ruled by Richard the Fearless, who was held captive as a child by Louis IV, but escaped and later introduced feudalism to it.  Through the treaty of Saint-Clair-sur-Epte, its first ruler was formally granted authority over it by Charles the Simple.  After Rollo, its rulers included Robert Curthose and Robert the Devil.  FTP, name this duchy in northwestern France which was also ruled by William the Conqueror.
This region is home to the island of Tatihou, which lies to the east of the Cotentin Peninsula.
Normandy