The dolorous stroke is the stroke that Sir Balin hit with King Pellam with. It caused three kingdoms to be laid to waste until the quest of the holy grail was completed. Sir Balin suffered because of this and it ultimately resulted in the killing of his brother. (1)



Kappes, Alfred (1850-1894). "How Balin Smote the Dolorous Stroke," from Lanier, Sidney.(2)


Speed, Lancelot (1860-1931), "Balin Strikes 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 laying waste to the land. Pellam 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.
Many people were rendered unconscious, including Balin. Some of them died in terror of the voice. Among those who had died was Balin's guide, the maiden who had accompanied him in his quest.

Above are more effects the dolorous stroke had upon the kingdom.
Sources:
(1) http://www.pantheon.org/articles/d/dolorous_stroke.html
(2) http://www.lib.rochester.edu/camelot/images/bkabal.htm
(3)http://www.timelessmyths.com/arthurian/excalibur.html