Eosa was a Saxon, one of the Germanic people from the coasts of northern Europe who had crossed the sea to Britain in the time of Vortigern, the British king who had invited them to fight the Picts and the Scots in exchange for land and tribute. He was a kinsman of Hengist, the Saxon warlord who had betrayed that trust so completely that his name became a byword for treachery in British legend. Some sources call Eosa a cousin of Hengist, others a son, and the Anglo-Saxon Chronicle identifies him with Æsc, a figure who may have been Hengist's son or his cousin. What is certain is that when Hengist brought his warriors to Britain, Eosa was among them, and when the Saxons turned from mercenaries to conquerors, Eosa fought at their side.

The first great test of Eosa's life came after the Battle of Conisbrough, where Aurelius Ambrosius, the last Roman king of Britain, had defeated the Saxon army and captured Hengist. Eldol, the Duke of Gloucester and the sole survivor of the massacre at Amesbury, had executed Hengist by beheading him before the assembled British lords. The Saxon host was shattered. Octa, the son of Hengist, retreated to York with what men he could gather, while Eosa led another contingent of survivors to a fortified town where he had stationed a bodyguard. Aurelius marched on York, and Octa, seeing the size of the British army, surrendered and begged for mercy. Aurelius held a council of his leaders, and Eldad, the holy Bishop of Gloucester and brother of Eldol, spoke in favour of clemency. The king granted it. When Eosa heard of this mercy, he came to Aurelius and begged for the same terms. A covenant was made: the Saxons would be allowed to inhabit the lands that bordered Scotland, on condition that they obeyed the King of the Britons, laid down their arms, and lived in peace.

For a time, the covenant held. Eosa and Octa settled their people in the northern marches, far from the centres of British power, and the island knew a measure of peace under Aurelius's forty-year reign. But then came the night at Winchester, when the false physician Eopa, a Saxon agent in the pay of Pascentius, poisoned Aurelius Ambrosius in his sleep. The king who had shown mercy was dead, and the covenant that bound the Saxons to peace died with him. Octa and Eosa, joined by the Saxon survivors of the army that Pascentius and Gillomanius of Ireland had raised, saw their chance. They broke the oath, took up their arms, and marched south, burning towns and cities as they came, laying waste to everything that stood in their path.

Uther Pendragon, the younger brother of Aurelius and the new king of the Britons, marched north to meet them. He found the Saxons besieging the city of York, and the two armies clashed in a battle so fierce that the Britons were forced to retreat. Uther and his men took shelter on a mountain crowned by a wood and split by a deep ravine, a place called Mount Damen. There, with defeat looming, Uther summoned his lords and captains to council. Gorlois, the Duke of Cornwall, proposed a bold plan: a night attack on the Saxon camp, taking the enemy by surprise while they slept. Uther agreed. Under cover of darkness, the Britons crept down from the mountain and fell upon the Saxon lines. The attack was a complete success. Octa and Eosa were captured alive, their army scattered, and the siege of York was lifted.

The two Saxon leaders were taken to London in chains and imprisoned there. Uther celebrated his victory with a great Easter festival in the city, wearing his crown before the assembled nobles of Britain to show that the kingdom was secure. But Octa and Eosa were not finished. They escaped from their prison, slipped out of London, and made their way back to Germany. There they raised a new army, gathering the Saxon youth with promises of gold and plunder, and returned to Britain with a fleet bent on conquest.

The final confrontation came at Saint Albans, the city the Romans had called Verulam. Octa and Eosa laid waste to the countryside around it, burning farms and villages, confident that the Britons could not stop them. But Uther Pendragon, though racked by illness, refused to yield. He was carried to the battlefield in a horse-drawn carriage, so sick that he could barely sit upright. When the Saxons heard that the half-dead king was leading the British army, they mocked him. They called him the half-dead king and said it would humiliate them to fight a sick man on his deathbed. In their pride and contempt, they left the gates of the city open. Uther attacked at once. The battle raged all day, and at nightfall both sides rested. At dawn the Saxons marched out to meet the Britons in open field, hoping to break through and escape. The fighting was bloody and fierce. At last, Octa and Eosa fell. Leaderless, the Saxon army broke and ran. Those who escaped headed north, raiding as they went, but they would never again threaten the south of Britain. Eosa, who had survived three wars, two imprisonments, and one desperate escape, died on the field at Verulam, his Saxon seax still in his hand.

Eosa disappears from the chronicles after Saint Albans, his name never spoken again in the histories of Britain. He is not a villain of the stature of Hengist, who butchered nobles at a peace feast, nor of Mordred, who betrayed a kingdom. He is something more ordinary and more telling: a warlord who accepted mercy, broke his oath, escaped prison, and kept fighting until there was nothing left to fight with. In Wace's Roman de Brut, the Anglo-Norman verse adaptation of Geoffrey's chronicle, Eosa appears under two names, Ebissa and Osa, and Layamon's Middle English Brut understood these as two separate characters, a confusion that speaks to the shadowy nature of the man. Whether he was Hengist's cousin, his son, or his grandson, the story is the same: a Saxon who would not stay defeated, who rose again and again from the ashes of each lost battle, and who finally found his end only when the half-dead king himself came to meet him.