Garlon was the brother of King Pellam, the lord of Listeneise, a castle and kingdom that existed on the border between the mortal world and the realm of the Holy Grail. Pellam was one of the keepers of the Grail, the vessel that had caught the blood of Christ at the crucifixion, and his castle was a place of such sanctity that it housed not only the Grail but also the Spear of Longinus, the weapon that the Roman soldier had used to pierce the side of the Saviour. Garlon, by contrast, was a man of darkness. He possessed the power to make himself invisible at will, a gift or curse that he used not for noble purpose but for murder. He would ride unseen through the forests and roads of Britain, spearing knights from behind, killing men who never saw their attacker and who died with confusion rather than honour on their faces.

The first recorded victims of Garlon's treachery were Sir Herlews le Berbeus and his lady. Herlews was riding with his damsel toward the castle of Meliot when Garlon, invisible and armed with a spear, struck him from behind. Balin, the Savage Knight, had been sent by King Arthur to protect Herlews on his journey, but the invisible attack came too swiftly for any defence. Herlews fell dead, and Balin, seeing the spear that had killed him, swore vengeance. The damsel who had ridden with Herlews told Balin the name of the murderer: Garlon, brother of King Pellam, the invisible knight who had destroyed many good men. Balin took up the quest, promising to avenge Herlews and to bring Garlon to justice.

On the road, Balin met a gentleman who was weeping at his own table. The man told a strange story. He had been at a tournament where he had twice overthrown a knight, and that knight had threatened to revenge himself by harming the gentleman's best friend. The knight had made good on his threat, wounding the gentleman's son with a blow that could not be healed except by the blood of the very knight who had dealt it. But the attacker had ridden invisible, and no one knew his name. Balin, hearing this, recognised the handiwork of Garlon. He told the gentleman that he knew the murderer's name and that he had already killed two companions of his own in the same base manner. He swore that he would rather meet Garlon than have all the gold in the kingdom, and he invited the gentleman to ride with him to Pellam's castle, where Garlon would surely be found.

The journey was marked by further carnage. As Balin, the gentleman, and a damsel approached a churchyard and hermitage, Garlon struck again. Sir Peryn, who had joined Balin's company, was speared by the invisible knight and fell dead before the hermit's door. The hermit told Balin what he already knew: that the murderer was Garlon, and that he could become invisible at will when he was armed. Balin pressed on, his fury growing with each death, until at last they reached the castle of King Pellam.

Pellam was holding a great feast, and Garlon was there, visible for once, sitting among the guests with a dark face that marked him as a man of ill temper. Balin entered the hall and took his place at the table, but he could not eat. His eyes were fixed on Garlon, and the hatred in them was plain for all to see. Garlon noticed the stare. In haughty anger he rose from his seat, walked over to Balin, and smote him on the face with the back of his hand. "Knight," he said, "why watchest thou me so? Eat thy meat, and do that thou camest for. Hereafter learn to use thy eyes to better purpose." It was the insult that Balin had been waiting for. He rose fiercely from his seat, drew his sword, and with one blow clove Garlon's head to the shoulders. "That is my errand here," Balin cried to the assembled guests. "With that truncheon thou killedst a good knight, and with this blow I revenge him." He called the gentleman forward to take Garlon's blood and heal his son, and the deed was done.

But the killing of Garlon was only the beginning of the catastrophe. King Pellam, seeing his brother fall dead before him, rose in fury and cried, "Why hast thou killed my brother! Villain and murderer, thou shalt die for this!" He snatched up a mighty weapon and struck at Balin, who threw up his own sword in guard. The blade shattered beneath the blow, leaving Balin unarmed. He ran from chamber to chamber, pursued by the enraged king, seeking any weapon he could find. At last he entered a richly furnished room where the Holy Grail stood upon a table of gold, and beside it lay the Spear of Longinus. A voice warned him not to touch it, but Pellam was close behind, sword raised for the killing stroke. Balin seized the spear and thrust it through both of Pellam's thighs. The king fell in a swoon, and at that moment the castle itself began to shake. The roof crashed down, the walls split asunder, and the great pile was reduced to rubble. Balin and Pellam lay buried beneath the ruins for three days, until Merlin came and freed them. The Dolorous Stroke had been delivered, and three kingdoms were laid waste by the enchantment that followed. Pellam's wound would not heal until Galahad, the purest knight in Christendom, achieved the Grail more than twenty years later.

Garlon's story does not end with his death. In Alfred Tennyson's Idylls of the King, the Victorian poetic cycle that reshaped Arthurian legend for the modern age, Garlon appears as a lover of Vivien, the enchantress who would later seduce and imprison Merlin. Tennyson gives him a blackened face and a mocking tongue, and in the idyll "Balin and Balan," Garlon insults Queen Guinevere within Balin's hearing, provoking the Savage Knight to attack him in the very hall where Pellam's feast is being held. The result is the same: Garlon dies, Pellam pursues Balin, and the Dolorous Stroke follows. But Tennyson's version adds a layer of psychological complexity, making Garlon not merely a murderer but a creature of malice who serves as the instrument of darker forces.