http://en.wikipedia.org/wiki/Enguerrand_II_of_Ponthieu

Enguerrand II was the son of Hugh II count of Ponthieu. He assumed the county upon the death of his father on November 20, 1052.
The Ponthievin alliance with duke William of Normandy had earlier been secured by the marriage of Enguerrand's sister, to duke William's uncle, William of Talou. Enguerrand was married to duke William's sister, Adelaide, by which he had 3 children. But for some consanguinity there, or from another infraction not now known, Enguerrand was excommunicated at the papal council held at Reims in October 1049.
William of Talou had built a strong castle at Arques, and from it he defied his nephew the youthful duke of Normandy: as "family", the comital house of Ponthieu supported the rebellion.
Duke William put Arques under siege, and then remained mobile with another force in the countryside nearby. He was aware that Normandy was being threatened by the armies of King Henry of France — who wanted to bring his young, former vassal to heel; and that Normandy's erstwhile allies from Ponthieu would also be coming to break the siege of Arques. Young count Enguerrand led a Ponthievin army of relief into the Talou and arrived first: but duke William successfully ambushed them on October 25, 1053 and Enguerrand was killed . Upon learning of this serious reverse, the vacillating Henry withdrew his forces at once back across the Norman border. William of Talou was compelled to surrender Arques and was banished for life.
Enguerrand's only son, Guy I became count of Ponthieu in his place.