Balan first appears in Thomas Malory's Le Morte d'Arthur, composed around 1469-1470, in the section Malory calls the Booke of Balyne le Saveage. Malory drew the tale from the Post-Vulgate Suite du Merlin, a French prose romance of the mid-thirteenth century, though the brothers' story is older than either text, rooted in the Celtic tradition of doomed twins. In Malory's telling, Balan is the brother of Balin, a poor knight from Northumberland imprisoned for killing one of King Arthur's cousins.

Balan found his brother on the road outside Camelot, not long after Balin had been banished for beheading the Lady of the Lake. Balin had just killed Sir Lanceor of Ireland in a joust, and Lanceor's lady Colombe had taken her own life upon his body. Balin stood weeping in the forest when Balan rode up, and the two brothers removed their helms and kissed each other. When King Mark of Cornwall happened by and asked their names, Balan answered for his brother, pointing to the two swords at Balin's side and saying that he might be called the Knight with the Two Swords.

The brothers set out to win back Arthur's favour by capturing King Rience of North Wales, who had made war upon Britain and demanded the beards of eleven conquered kings. With Merlin's guidance they ambushed Rience, slew forty of his men, and delivered the captured king to Camelot. Arthur marvelled at their prowess and called them the best knights he had ever seen. They fought again at the battle of Terrabil against the rebel kings Nero and Lot, where their strokes were so wonderful that men said they came from heaven or from hell. Merlin prophesied that Balan would depart and never return to Arthur's court.

Balan rode off alone and came to an island castle guarded by a bridge. There he encountered a fierce knight in red armour who challenged all who passed. Balan slew the knight, but in doing so he fell under the same curse: he was now condemned to remain on the island as its champion, challenging every traveller who sought to cross. He could not leave, could not send word to his brother, could only wait in red armour for the next knight to come.

That knight was Balin. After the Dolorous Stroke that laid waste to three kingdoms, Balin came to the same castle and was told he must joust the guardian before he could pass. A knight offered him a larger shield to replace his own, which bore the device of the Two Swords, and Balin accepted. He crossed to the island, where a damsel warned him that he had put himself in great danger. From the castle rode a knight in red armour. Balan looked at the two swords and thought he knew the man, but the strange shield deceived him. They couched their spears and rode together with such force that both horses went down. They fought on foot for hours, wounding each other grievously, until the ground was red with their blood. At last Balan withdrew and lay down. Balin asked his name, and when Balan answered, Balin fell backward in a swoon. Balan crawled to his brother and removed his helm, but the face was so scarred that he did not know it until Balin woke and spoke. They wept together, cursed the castle's ill customs, and asked the lady to bury them in one tomb. Balan died first, and Balin followed at midnight. The lady buried them richly, writing Balan's name upon the stone, for she did not know his brother's. In the morning Merlin came and added Balin's name in letters of gold.