Even the year is uncertain. Sir Robert Gordon, the historian who lived from 1580 to 1656 and wrote the Genealogical History of the Earldom of Sutherland, placed the battle around 1464. The book Conflicts of the Clans, first published by the Foulis Press in 1764 but drawn from a manuscript written during the reign of James VI, gave the date as 1438 - a quarter-century earlier. Modern historians lean toward 1464 but treat both dates as possibilities. What is not disputed is the cause. Disputes had risen between Keith of Ackergill and the Gunns, who were the dominant clan of Caithness, and the Keiths felt they could not win on their own. So they sent for the Mackays of Strathnaver.

When the Mackays arrived under their chief Angus, son of Niel-Wasse, the inhabitants of Caithness convened in haste. The Gunns and their allies - possibly the Clan Oliphant and the Clan Sutherland - met the combined Keith and Mackay force on the moor of Tanach. Sir Robert Gordon's account is blunt: "There ensued a cruel fight, with great slaughter on either side." In the end the Keiths had the victory, and Gordon credits one man above all others for the outcome: John-More-MacK-Ean-Reawich, an Assynt man whose valour and manhood, Gordon wrote, became famous throughout these countries. The Conflicts of the Clans version names the same warrior, calls him John Mor MacIan-Riabhaich, and adds the detail that two chieftains and leaders of the Caithness men were killed in the fighting. The Gunns lost the battle, but not the war - the feuds rumbled on, with the Battle of Champions still to come a decade or so later.

Angus Mackay survived Tannach but did not survive his own enemies. The Conflicts of the Clans manuscript records his end with characteristic medieval directness: he was burnt and killed in the Church of Tarbat by a man of the surname of Ross, whom Angus had molested repeatedly with incursions and invasions. The Mackays were one of the great northern clans of Strathnaver, a power in their own right, and their willingness to ride into Caithness on behalf of the Keiths shaped this corner of Scotland for decades. The Moor of Tanach itself, southwest of Wick, is now ordinary farmland and rough pasture - no monument marks the battle, and the precise location of the slaughter is uncertain. But the stories survive in the old books, written down centuries after the swords had rusted, by men who had inherited the names if not the grudges of the dead.