Wikipedia (from which the image is copied) says:

Hugh tried to starve himself before his trial, but face trial he did on November 24, 1326, in Hereford. He was judged a traitor and a thief, and sentenced to public execution by hanging, drawing and quartering. Being a traitor was also what Gaveston had been executed for as the belief was that these men had misled the King rather than the King himself having created the folly. Immediately after the trial, he was dragged behind four horses to his place of execution at Hereford, where a great fire was lit. The queen watched, with Mortimer, as Despenser was stripped naked, swung up in the air by a noose around his neck, and hanged from a ladder fifty feet high, but cut down before he could choke to death and tied to a ladder, in full view of the crowd. A man climbed up beside him, and sliced off his penis and testicles which were then burnt before him, still alive and conscious. Subsequently, the executioner plunged his knife into his abdomen, and cut out his entrails and heart, which were likewise burnt before the delighted crowd. Finally, he was beheaded, and his body cut into four pieces, and his head was mounted on the gates of London.

No book-length biographical study of Hugh Despenser exists, although The Tyranny and Fall of Edward II: 1321-1326 by historian Natalie Fryde is a study of Edward's reign during the years that the Despensers' power was at its peak. Fryde pays particular attention to the subject of the Despensers' ill-gotten landholdings. The numerous accusations against the younger Despenser at the time of his execution have never been the subject of close critical scrutiny, although historian Roy Martin Haines called them "ingenuous" and noted their propagandistic nature.

Despite the crucial and disastrous role he played in the reign of Edward II, Despenser is almost a minor character in Christopher Marlowe's play Edward II, where as "Spencer" he is little more than a substitute for the dead Gaveston.

According to Alison Weir (Isabella, She-Wolf of France, Queen of England, Pimlico, 2006, 240)

"Despenser... at first suffered with great patience, asking forgiveness of the bystanders, but then a ghastly, inhuman howl broke from him".