The Dolorous Stroke is a trope in Arthurian legend and some other stories of Celtic origin.
Sir Balin left the court of King Arthur on his quest to avenge a knight he had failed to protect. Balin met another knight whom he befriended. Merlin met them in another disguise, revealing the identity of the invisible assassin was Garlon the Red, the brother of King Pellam. Merlin insisted that Balin should give up the quest or else bring sorrow and misfortune to Logres. Balin stubbornly refused to give up on the quest to avenge the unnamed knight he had failed to protect.
When Balin and his new companion entered the cemetery, the invisible knight struck again, mortally wounded his new friend. Balin encountered a monk and found out that Gawain would avenge his father upon Pellinor. After a few day of travel they reached the castle of King Pellam. Pellam was holding a feast on Sunday. Balin found out that Garlon the Red can become invisible at will, when he was armed. Balin pondered if he should attack Garlon when Garlon was unarmed but that would branded Balin as a coward or waited until Garlon was armed, making difficult for him when he can't see his opponent. As Balin thought of his dilemma Garlon noticed Balin was not eating the food, as a guest should. Garlon struck Balin's face with his gloves, for being rude. Balin answered by cutting off Garlon's head with swipe of his sword.
Seeing Garlon fall, Pellam went to avenge his brother, shattering one of Balin's swords. Balin went from room to room to find another weapon. Balin entered one room where the Holy Grail and the Spear of Longinus were kept. A voice warned him not to enter. Balin did not heed the voice since Pellam was pursuing closely behind Balin. Despite hearing the voice, warning the hero not to touch the spear, Balin seized the lance and ran the weapon through both of Pellam's thighs. It was this strike that was called the "Dolorous Stroke". A voice was heard throughout the castle that a great enchantment would fall upon Logres.
The Dolorous Stroke had not only wounded a king, but set off an earthquake and Pellam, the Maimed King was seriously wounded and his wound would not heal until the Grail knight (Galahad) healed him twenty-two years later, at the end of the Grail quest. Merlin arrived two days later and found that the people outside of the castle were too frightened to enter the castle to see if anyone had survived. Merlin went into the castle and found Balin lying there unconscious for two days. Merlin brought Balin out of the castle. Merlin showed the devastation of the three kingdoms that Balin had caused with that one stroke of the lance.
Balin had been warned several times by Merlin and other people that this would happened. Balin had heedlessly sought glory and fame as a knight, only to bring sorrow to so many.