As the alliance of such a powerful and war-like chief as Mackay would have been of great importance to the Sutherland interest, an attempt was made to detach him from the Earl of Caithness. The plan appears to have originated with Hugh Murray of Aberscors, who made repeated visits to Strathbogie, to consult with the Earl of Sutherland and his friends on this subject, and afterwards went into Strathnaver and held a conference with Mackay, whom he prevailed upon to accompany him to Strathbogie. Mackay then entered into an engagement with the Earl of Huntly and the Earl of Sutherland, to assist the latter against the Earl of Caithness, in consideration of which, and on payment of £300 Scots, he obtained from the Earl of Huntly the heritable right and title of the lands of Strathnaver; but Mackay, influenced by Barbara Sinclair, the wife of the Earl of Sutherland, with whom he now publicly cohabited, broke his engagement, and continued to oppress the earl’s followers and dependents. From some circumstances which have not transpired, the Earl of Caithness became suspicious of his son John, the Master of Caithness, as having, in connection with Mackay, a design upon his life. To put an end to the earl’s suspicion, Mackay advised John to go to Girnigo (Castle Sinclair), and to submit himself to his father’s pleasure, a request with which the Master complied; but, after arriving at Girnigo, he was, while conversing with his father, arrested by a party of armed men, who, upon a secret signal being given by the earl, had rushed in at the chamber door. He was instantly fettered and thrust into prison within the castle, where, after a miserable captivity of seven years, he died, a prey to famine and vermin.