The feud's origin story is grim even by the standards of medieval Caithness. Dugald, chieftain of the Keiths, discovered that Helen of Braemore - daughter of Lachlan Gunn - was betrothed to Alexander Gunn. On the night before the wedding, the Keiths attacked. They abducted Helen and killed Alexander among others. Helen subsequently threw herself from Ackergill Tower rather than live with her captors. From that single, terrible night the two clans entered decades of indecisive raiding, with casualties on both sides and no resolution. By the late 15th century both clans were exhausted enough to try diplomacy. They agreed on a trial by combat at the chapel of St Tayre, on the coast just south of where Castle Sinclair Girnigoe now stands. Twelve men on each side, all on horseback. A settlement by blood, but a settlement.

The account survives in two old books. The first is Conflicts of the Clans, published by Foulis Press in 1764 from a manuscript written during the reign of James VI. The second is Robert Mackay's History of the House and Clan of the Name Mackay, published in 1829, which quotes the historian Sir Robert Gordon (1580-1656). Both tell the same essential story. The Crowner of the Gunns - the clan chieftain - arrived first with his eleven kinsmen and went into the chapel to pray. The Keiths came late. When they appeared, there were twelve horses, as agreed, but twenty-four riders: two men on each mount. The Laird of Inverugie and Ackergill argued it was no breach of agreement - they had twelve horses, exactly as bargained for. The twenty-four Keiths burst into the chapel and fell upon the Gunns at their prayers. The fight was savage. Eight of the Gunns died. Most of the Keiths died too. Sir Robert Gordon wrote that the blood was still visible on the chapel walls in his own century.

The crowner's grandson, William - called Mackames or Mackamish, meaning "son of James" - led the response. James Gunn, the crowner's son, had left Caithness with what remained of his clan and settled in Sutherland, where they became wardens of the district. William grew up in Sutherland with his father's grievance. Years later he intercepted George Keith of Ackergill on the road from Inverugie to Caithness, with George's son and twelve of their followers, and killed every one of them. From that moment, the chief of Clan Gunn carried the title Mackamish in honour of William's revenge. The feud guttered on, never quite reignited, never quite resolved, for the next five centuries. Both clans dispersed; many emigrated to North America; the original cause was forgotten by all but the antiquarians.

By the 1970s, with descendants of both clans on friendly terms in Canada and the United States, the feud had become a curiosity rather than a wound. On 28 July 1978, at the site of St Tears chapel on the 500th anniversary of the massacre, the Chief of Clan Keith - Sir James Ian Keith, Earl of Kintore - and the Commander of Clan Gunn - Ian Alexander Gunn of Banniskirk - signed a Bond and Covenant of Friendship. It is one of the longest formally ended feuds in recorded history. Members of both clans still mark the treaty at Highland games and other gatherings, raising glasses to a peace that took half a millennium to achieve - and to the eight Gunns and most of the Keiths who died at prayers.