Esclabor's story begins in the east, in Babylon or Galilee, depending on the version, where he ruled as a pagan king before fate and diplomacy sent him westward. He journeyed to Rome as a tribute from his homeland, a royal hostage or ambassador in the ancient tradition of empire, and while there he performed a deed that would define his legend: he saved the life of the emperor. The details are lost in the fragments of the romance that bears his son's name, but the act was great enough to earn him honour in the greatest city of the medieval world. Yet Rome was not his destination. He travelled onward, across the sea and into the misty island of Britain, where he found another king in peril and repeated his salvation. King Pellinore, the lord of Listenoise and the hunter of the Questing Beast, lay in danger, and Esclabor drew his sword and rescued him. This second saving of a king bound him to Britain forever, and he entered the service of King Arthur, the young monarch who was building the fellowship of the Round Table.

Arthur welcomed the foreign king, but a misunderstanding would give Esclabor the name by which history remembers him. The King of Britain had assumed that Esclabor was a Christian, as so many who came to his court claimed to be, and when the truth emerged - that Esclabor was a heathen, a pagan from the Saracen lands - Arthur bestowed upon him the epithet "the Unknown." It was not a name of mockery but of mystery, a recognition that this king from the east carried a faith that Arthur's court did not understand. In time, Esclabor was baptised, joining the religion of the land that had become his home, and his conversion opened the door for his children to enter the highest circles of Arthurian chivalry. He had a brother named Aphasar, called Aphasar the Unknown, who shared in the family's mysterious reputation, and together they represented a bridge between the pagan east and the Christian west that Arthur's fellowship was meant to unite.

Esclabor's family was large and ill-fated. He fathered Palamedes, the most famous of his sons, the Saracen knight who would become the hunter of the Questing Beast and the rival of Tristan for the love of Iseult. He fathered Safir, a knight of such prowess that he killed the Count of the Plank, an enemy who had threatened his father's life. He fathered Segwarides, who joined the Round Table and fought alongside the greatest knights in Christendom. He fathered a daughter named Florine, a maiden whose name appears in the lists of Arthurian ladies. And he fathered nine to eleven other sons, perhaps as many as fourteen children in all, a royal brood that should have ensured his lineage for generations. But fate was cruel. All of these other sons, save Palamedes, Safir, and Segwarides, were slain by the Questing Beast, the monstrous chimera with the head of a serpent, the body of a leopard, the haunches of a lion, and the feet of a hart, whose belly issued the sound of thirty couple hounds questing. The beast that Pellinore had hunted became the destroyer of Esclabor's house, and the king who had saved Pellinore's life watched his own children fall to the creature his friend had pursued.

The Grail Quest brought Esclabor his finest hour. During the great search for the Holy Grail, when the knights of the Round Table dispersed across Britain and beyond to seek the sacred vessel, Esclabor rode with his son Palamedes and with Galahad, the perfect knight who alone would achieve the Grail. Together they faced one of the gravest threats to Arthur's kingdom: King Mark of Cornwall, the treacherous monarch who had married Iseult and who now invaded Logres, Arthur's realm, while the king's greatest knights were absent on the quest. Esclabor, Palamedes, and Galahad helped repel Mark's first invasion, defending the kingdom that had given them refuge against the very king who had driven Tristan into exile and who would one day be remembered as one of the villains of the Arthurian world. It was a moment of unity across generations and faiths: the pagan king turned Christian, his Saracen son, and the purest knight in Christendom fighting side by side for the land they all served.

But the Arthurian world is a place of tragedy as much as triumph, and Esclabor's end was the darkest of all. In the Post-Vulgate Queste del Saint Graal, the version of the Grail Quest that emphasises the spiritual and the pessimistic, Palamedes was slain by Sir Gawain. The circumstances are bitter: Gawain, once a friend to Palamedes, was obliged to kill him because Palamedes had killed King Mark, who had killed Tristan; and King Mark had been provoked by the sinister Mordred to kill Tristan with Palamedes' own spear. The web of vengeance was so tangled that no sword could cut it clean. When Esclabor learned that his son was dead, that the knight he had raised and loved, the last great hope of his lineage, had fallen to Gawain's blade, the grief was more than he could bear. He committed suicide, ending his own life in the despair of a father who has outlived too many children. The Post-Vulgate Cycle, composed between 1230 and 1240, records this ending with the bleakness that characterises its vision of the Arthurian world: even the conversion of the pagan, even the bravery of the father, even the service to Arthur, could not protect Esclabor from the final tragedy that awaits all who love too deeply in a world built upon blood feuds and doomed fellowship.