Fionn mac Cumhaill is leader of the Fianna knights of legend and lord of Diarmuid Ua Duibhne, the Lancer of the 4th Holy Grail War.

He was the intended husband of Princess Grainne, whom Diarmuid inadvertently charmed with his Mystic Face. He was thus forced to elope with her, and Fionn, driven by jealousy, sent many pursuers after them. After some times, Fionn acknowledged their marriage, and reconciled with Diarmuid. Still, his resentment remained, and eventually drove him to leave Diarmuid for death when the warrior was fatally injured by a boar during their hunting trip.

Even after his death, Diarmuid resented neither Fionn nor Grainne, and regretted only that he could not follow his path of loyalty to Fionn. For this reason, Diarmuid participated in the War, in order to have a second chance of fighting for his master.