Grimoult was part of a conspiracy to assassinate William the Bastard, who at that point was struggling to maintain control of the duchy of Normandy after the death of his father. In 1047 the conspirators arranged for William, then only 19, to be killed at Valognes, but William was warned and the assassination attempt failed. Grimoult and his co-conspirators raised an army of around 25,000 men and fought against William and his backer King Henry I of France at the Battle of Val-ès-Dunes. Because Grimoult held less influence than the other conspirators he was the only one to be imprisoned in the tower of Rouen and then executed by the Duke. Grimoult's lands were confiscated, his castle was destroyed and his barony was given to the Duke's half brother Odo, the Bishop of Bayeux, for the use of the church.