Eldol was the Count or Duke of Gloucester, a city in the west of Britain that would one day give its name to the county that surrounds it. He was the brother of Eldad, the holy bishop of Gloucester who would later bury the victims of the massacre and speak the last rites over their bodies. In the time before the Saxon invasion, Eldol served as one of the leading nobles of the realm under Vortigern, the king who had invited the Saxons into Britain to help fight the Picts and the Scots. Vortigern had taken Rowena, the daughter of the Saxon warlord Hengist, as his wife, and in doing so he had bound the fate of Britain to the ambitions of a people who had come as mercenaries but stayed as conquerors.

The defining catastrophe of Eldol's life came at a peace conference on Mount Ambrius, near what is now called Salisbury. Hengist, seeing that the Britons were growing wary of his people's increasing power, proposed a treaty to settle the matter once and for all. Vortigern agreed, and a great banquet was arranged at the monastery founded by Ambrius, where three hundred monks still lived and prayed. The condition was strict: no man, Briton or Saxon, was to carry a weapon to the feast. The Britons obeyed. The Saxons did not. Hengist had ordered every one of his chieftains to conceal a long knife, a seax, beneath his clothing. When the wine was flowing and the Britons were at ease, Hengist gave the signal, a shout in the Saxon tongue: "Nemet oure Saxas!" - Draw your knives! The Saxons pulled their blades and fell upon the unarmed nobles beside them. Four hundred and sixty counts and earls were cut down where they sat. Vortigern alone was spared, held fast by his royal robe so that Hengist could ransom him later.

Eldol was there, unarmed and surrounded by enemies. He looked about him and saw a wooden stake lying on the ground, perhaps a billet of firewood or a post from the monastery fence. He seized it and began to fight. For the space between the setting of the sun and the coming of darkness, he laid about him with that stake, crushing skulls and breaking arms, killing seventy Saxons by his own hand and disabling many more before the press of bodies grew too thick. In the chaos he fought his way to a horse, mounted it, and galloped through the lines to Gloucester, his own city. He was the only British noble to escape alive. The event became known as the Treachery of the Long Knives, and it left Britain virtually bereft of leadership, with Hengist holding Vortigern as a puppet and the Saxons in control of the fortified places of the realm.

Eldol did not hide in Gloucester. He gathered what men he could and waited for the return of the rightful heirs. When Aurelius Ambrosius and his brother Uther sailed from Brittany with a fleet of Britons and Bretons, landing at Totnes, Eldol was among the first to join them. He told Aurelius the full story of the massacre, how he had survived, and how Vortigern had allowed it to happen through his foolish trust in Hengist. Aurelius, who had come to avenge the murder of his father Constantine and his elder brother Constans, turned first upon Vortigern. Eldol rode with him to Ganarew, where the traitor king had built a tower as his last refuge. When siege engines failed to breach the walls, Aurelius ordered the moat drained and filled with wood and combustible material. Archers fired burning arrows into the stronghold, and Vortigern perished in the flames along with his wives and followers. Eldol watched the tower burn, knowing that the man who had brought the Saxons into Britain was dead.

Then came the battle that Eldol had been waiting for since the night of the long knives. Aurelius marched north to confront Hengist, who had regrouped his Saxons at a place called Kaerconan, which some identify with Conisbrough. Hengist, seeing that he could not avoid battle, arrayed his men and prepared to fight. The clash was terrible. Aurelius sent his cavalry against the Saxon lines, breaking their ranks and causing confusion. Eldol, fighting on foot at the head of the infantry, cut his way through the enemy host, killing all who stood before him but always searching for one man. At last he found him. Hengist, the warlord who had betrayed four hundred and sixty nobles at a peace feast, stood before him. The two fought a ferocious duel, sword against sword, Hengist's skill matched against Eldol's fury. Hengist drove forward, then Eldol parried and struck back. The battle raged around them until a fresh contingent of cavalry led by Gorlois, the Duke of Cornwall, arrived and gave the Britons new heart. Eldol surged forward, seized Hengist by his war helmet, and dragged the Saxon chieftain into the British ranks. The Britons clamoured for Hengist to be killed on the spot, but Eldol refused. He declared that the warlord would face the justice of King Aurelius, not the sword of the battlefield.

The council that judged Hengist was held before the king and his lords and bishops. Eldad, the holy bishop of Gloucester and Eldol's own brother, stood and spoke. He compared Hengist to Agag, the king of the Amalekites whom the prophet Samuel had hewn to pieces, saying that as Hengist's sword had made women childless, so should his mother be made childless among women. The council agreed, and Eldol himself led Hengist from the chamber and executed him, beheading the man who had betrayed a nation. Aurelius, showing respect even to a fallen enemy, ordered that earth be raised over Hengist's body in the Saxon manner of burial. Eldol had his vengeance, and the Saxon threat was ended, at least for a time.

Eldol's story does not end with the execution. He appears again in the Elizabethan play The Birth of Merlin, attributed to William Shakespeare and William Rowley, where he serves as a general under Aurelius. In Welsh tradition, recorded in the Triads compiled by Iolo Morganwg, he became Eidiol 'Gadarn', one of the Three Vigorous Ones of Britain, alongside Gwrnerth of the sharp shot and Gwgawn of the mighty hand. The Welsh version of his escape claims that he slew not seventy but six hundred and sixty Saxons with his wooden billet, a number that speaks to the growing legend of the man. He should not be confused with an earlier King Eldol who appears in Geoffrey's chronicle, a figure from generations before who fought a different war. The Eldol of Gloucester stands apart, the man who would not die at a treacherous feast, who would not rest until the butcher of Amesbury faced justice, and whose name became a byword for survival against impossible odds.