Robert Curthose
Eustace was the son of Count Eustace II and Ida of Lorraine. In 1088, he rebelled against William II of England in favour of Robert Curthose. While waiting for Robert Curthose's arrival from Normandy, Eustace and his fellow compatriots were besieged at Rochester castle by William II. With provisions running out and the situation becoming dire within the castle, the rebels asked for terms. William II pardoned most of the rebels allowing those such as Eustace to return to Normandy. In 1091, Eustace was with Robert Curthose when the latter agreed to terms with William II, recognizing him as king of England.