The Barons de Braose

At his peak he was also Lord of Gower, Abergavenny, Brecknock, Builth, Radnor, Kington, Limerick, Skenfrith, Grosmont and Whitecastle.

William de Braose, 7th Baron Abergavenny and 4th Lord of Bramber (1140/1150 - August 9 1211) at his peak was also lord of Gower, Abergavenny, Brecknock, Builth, Radnor, Kington, Limerick, Skenfrith, Briouse Castle in Normandy, Grosmont, and White Castle. His rise and fall at the hands of king John is often taken as an example of that king's arbitrary and capricious behavior towards his barons.
William was the son of William de Braose, Third Lord of Bramber and Bertha of Hereford, daughter of Miles Fitz Walter, Earl of Hereford and his wife, formerly Sibyl de Neufmarche. From his father he inherited the Rape of Bramber, in Sussex, and through his mother he inherited a large estate in the Welsh Marches.

In 1175, William carried out the Massacre of Abergavenny, killing several Welsh princes to avenge the death of his uncle Henry, Earl of Hereford, after having invited them to a feast at Abergavenny Castle. This resulted in great hostility against him among the Welsh, who named him the "Ogre of Abergavenny".

In 1199, William fought beside King Richard the Lion-heart at Chalus, where Richard was killed. He was greatly favored by King John early in his reign. John granted him all that he might conquer from the Welsh in Radnor, gave him lordship over Limerick in Ireland (save for the city itself), possession of Glamorgan castle, and then lordship over Gower.

In 1203, William was put in charge of Arthur of Brittany, whom he had personally captured the previous year. William was suspected of involvement in Arthur's disappearance, although no concrete evidence ever came to light. There is somewhat better evidence that he at least knew the truth of the matter.
In 1206 John gave William the three great castles of Gwent, Grosmont, and Whitecastle. At this point only an earldom separated him from the greatest in England. But soon after William fell out of favor with the king. The precise reasons remain obscure. John's stated reasons regard money de Braose owed the crown. But the king's actions went far beyond what would be necessary to recover the debt. Instead, he evidently wanted to break de Braose, and to that end invaded Wales to seize the de Braose domains there. Beyond that, he sought de Braose's wife, who, the story goes, had made no secret of her belief that John had murdered Arthur of Brittany.

De Braose fled to Ireland, then returned to Wales as John hunted him in Ireland. In Wales, William allied himself to the Welsh prince Llywelyn the Great and helped him in rebellion against King John.
In 1210, William fled in disguise to France and died the following year at Corbeil. William's wife, Maud de St. Valery, and eldest son, William, were captured and murdered by King John, possibly starved to death.

While William had aroused the jealousy of the other barons during his rise, the arbitrary and violent manner of his fall very likely discomfited them and played a role in the baronial uprisings of the next decade. The historian Sidney Painter, in his biography of King John, called it "the greatest mistake John made during his reign, as the king revealed to his barons once and for all his capacity for cruelty".
Eventually, William's third son, Reginald de Braose reacquired some of his father's titles and lands. The middle son, Giles, was Bishop of Hereford from 1200 until his death in 1215.

William's eldest daughter Matilda (also called Maud) married Gruffydd ap Rhys II of Deheubarth. Another daughter, Margaret, married Walter de Lacy, Lord of Meath.