It's a moment that I dramatize in A Place Beyond Courage, but here's the primary source.
"About this time Robert FitzHubert a Fleming by race, deceitful in mind and deed, cool, who as is said of the judge in the Gospel, regarded neither God nor men, stole away from Earl Robert's army (for he was in his pay) and by stretching from the parapet of the wall to the ground scaling ladders strongly and skilfully made of leather took by night a castle of the King named Devizes which was finely and impregnably walled.
And when he had brought himself and his men in, eluding the guards, he seized all the King's partisans who were within, enjoying untroubled sleep and suspecting no such attempts, except that a few in the dead of night, when a shout went up from the storming party, hastily took refuge in a very high tower; but as they had taken no food with them and no help came from the King they surrendered the tower in a few days.
Then, when the news of such a remarkable feat was spread abroad, the Earl of Gloucester sent his son with a large force, as if to aid Robert's enterprise; but Robert, driving him away from the gates with insulting language, intermingled with threats, sent him back to his father in contumely, saying he had captured the castle to occupy it, not to hand it over to one stronger than himself.
Such indeed was the fact, such the cunning intention of that turncoat, not to keep on the Earl's side, nor to proclaim himself a supporter of the King, but to bring into the castle a large body of his own people and either ensnare by craft or seize by force all the surrounding country.
But by a wonderful judgement of God his wickedness returned upon himself, since according to a maxim inspired by God 'Wherewith a man sins, thereby shall he be likewise tormented.' For because he craftily tried to entrap others he too was entrapped by craft, captured and chained and died worn out by divers tortures.
There was in the neighbourhood a certain John, a man equally cunning and very ready to set great designs on foot by treachery, in forcible possession of a very strong castle named Marlborough. Robert was anxious to gain possession of it, either because it was near his own castle and conveniently situated, or because if that too were brought under his power he could more freely cause discord in the whole of England.
He sent word to John by intermediaries that he would make a pact of peace and friendship with him, that he wanted to ask admission to his castle for the sake of giving and receiving advice, that it was his intention to keep the pact unbroken and their harmony unimpaired.
But John, perceiving that he made all these promises in the hope of surprising the castle (which was the fact), gladly and affably agreed to his requests, and after admitting him to the castle shut the gates behind him and put him in a narrow dungeon to suffer hunger and tortures, and falling with his men on Robert's companions, whom he had brought with him as accomplices of his treachery, he laid hands on some at once, captured them, and imprisoned them with their leader. Others he put to shameful and discreditable flight, and compelled them to retreat all the way to Devizes.
The Earl of Gloucester, hearing that the very evil man was thus kept in the custody of John, who was still a most loyal supporter of his own party, rejoiced not a little, and summoning a very numerous body of knights to accompany them he came to John, and bringing Robert in front of Devizes he hanged him on high before the eyes of all his men and it was the most righteous vengeance of God in as much as he had afflicted many thousands of men with torments."
Harsh time indeed
himself ended his life with the torment he deserved.
Harsh times indeed.