Hugh spent much of his time fighting with his neighbours in Wales. Together with his cousin Robert of Rhuddlan he subdued a good part of northern Wales. Initially Robert of Rhuddlan held north-east Wales as a vassal of the tenant-in-chief. In 1081 Gruffudd ap Cynan, King of Gwynedd was captured through the treachery of one of his own men at a meeting near Corwen. Gruffudd was imprisoned by Earl Hugh in his castle at Chester, but it was Robert who took over his kingdom, holding it directly en liege from the king. When Robert was killed by a Welsh raiding party in 1093 Hugh took over these lands, becoming ruler of most of North Wales, but he lost Anglesey and much of the rest of Gwynedd in the Welsh revolt of 1094, led by Gruffudd ap Cynan, who had escaped from captivity.
Norwegian invasion
In the summer of 1098 Hugh joined forces with Hugh of Montgomery, 2nd Earl of Shrewsbury, in an attempt to recover his losses in Gwynedd. Gruffudd ap Cynan had retreated to Anglesey, but then was forced to flee to Ireland when a fleet he had hired from the Danish settlement in Ireland changed sides. Things were altered by the arrival of a Norwegian fleet under the command of King Magnus III of Norway, also known as Magnus Barefoot, who attacked the Norman forces near the eastern end of the Menai Straits. Earl Hugh of Shrewsbury was killed by an arrow said to have been shot by Magnus himself. The Normans were obliged to evacuate Anglesey altogether leaving Gruffydd who had returned from Ireland to take possession the following year. Hugh apparently made an agreement with him and did not again try to recover these lands.
Marriage, death and succession
Owing to his gluttony, Hugh became so fat that he could hardly walk, earning him the nickname of le Gros. He would also earn the nickname Lupus for his savage ferocity in battle against the Welsh, after he was dead. Hugh d'Avranches married Ermentrude of Claremont, daughter of Hugh I, Count of Clermont-en-Beauvaisis. Hugh and Ermentrude had the following children:
Hugh is also credited with siring children to many mistresses. His allegedly illegitimate children include:
Robert d'Avranche
Ottiwel d'Avranches, governor and tutor to King Henry's sons, married to Marguerite, daughter of Eudo Dapifer, steward to William the Conqueror and Henry I. They had one son William. Ottiwel died in the wreck of the White Ship.
He received many of the local manors held by Edwin the last Saxon Earl of Mercia. Edwin was the grandson of Leofric, Earl of Mercia. Leofric had been a holder of the Saxon title "Earl of Chester". Hugh fell ill and became a monk in July 1101. He died four days later and was buried in the cemetery of St. Werburgh. He was succeeded as Earl of Chester by his son Richard, who married Matilda of Blois, a granddaughter of William the Conqueror. Both Richard and Matilda died in the White Ship disaster, and Hugh was then succeeded by his nephew Ranulph le Meschin, Earl of Chester, son of his sister Margaret by her husband Ranulf de Briquessart, Viscount of the Bessin. His nephew removed his remains to have them reburied in the Chapter House ofChester Abbey.