The Feinne came over to Scotland from Ireland and they drove out the Scandanavians, who fled away to sea. When Alba was won, they debated going back to Ireland, but Cumal said no. "I say that if you reach Ireland the king would rather see you burned on a hill than face you. He could could not keep you there. Better keep the realm you have won; make your schemes and plans; make a king of the best man, and let us stay where we are."
Now the Feinne were all of one kindred and blood, and they themselves did not know who was chief amongst them. They sought amongst themselves to find the man of the best head. Cumal himself was the best at answering questions and making plans and he had the blood of kings running in his veins. Therefore, they chose him to be Rìgh Na Fèinne, king of the Feinn.
One of the Fein, a man named Arc Dubh (Black-Black), had committed a grievous crime and for that cause was expelled from the Feinn. He offered his services to the King of Ireland, boasting that he could fight a hundred men, all by himself (even the least man of the Feinn could fight a hundred) but that he would need food to match.
The King refused him, he couldn't afford to keep him, but Arc Dubh argued long and hard. When the King found that Arc Dubh could fish, he said, "I have the best river in all these realms, Eas Ruadh (Red River or Red Cascade, the Assaroe River in Sligo): go and get married and be there. Two-thirds of the fish you catch shall be mine. One-third shall be yours, and wages to boot, and so may you keep yourself."
Arc Dubh took this offer and so was called "the king's fisherman". (He is remembered as the fisher of Conn of the Hundred Fights.) He is also remembered as Hagen, who slew the hero Siegfried.
Now the Feinn who were in Scotland wrote to the King of Ireland to tell him that they meant to keep the realm they'd won. The King was not happy and he wished no one had ever thought of the Feinn. He wrote to the King of Lochlann, suggesting that they two join forces to make a plan to get rid of Cumal and his warriors. The king of Lochlann and his son came to Ireland for the talks and the King of Ireland greeted them warmly - all because of the Feinn. The two kings got to talking.
"They say that none in this world are like them," said the Scandinavian king. "I should like to see them."
"I have one of them here," replied the Irish king. "He will soon come with fish."
Early in the morning came Arc Dubh to the palace, before the Lochlannaich king was up. As soon as he heard that the warrior was come, though, the king leapt up and ran out half-dressed to meet him.
"Is this a Fiantaiche?" he asked.
"That title and order is lost," replied the fisher. "Perhaps it was my own fault, but I was one among the Feinn once."
"If all the others are like to you, then they are a wonderful and terrible people," the Lochlannach exclaimed. Arc Dubh agreed with him that this was so. "What tale can you tell of them?"
"I can tell this. There is one amongst them, their king, who is called Cumal. If all that there ever were, or who have come, or who will come were to go up against him, he would come through them readily with his sword." Arc Dubh's words dismayed the two kings.
"Will he be so till death comes to seek him?" persisted the Lochlannach. "or can he be slain?"
"No. I have sworn not to tell that secret."
The word he used was diamhaireachd, which means a concealed, hidden secret. To give a well-known example, The diamhaireachd of the Greek hero Achilles was that of all the places he could be struck, on a wound to his heel could kill him. Each of the Feinn had such a secrets they were sworn never to reveal any of these to anyone.
None the less, when threatened with death if he did not so tell, Arc Dubh decided that it was easier to be forsworn than to die.
"His death is his own sword, Mac a Luinne ("son of anger"/ "son of the sword"), and that will only slay him in the arms of his wife."
The King of Lochlann smiled upon hearing this. "I have the most beautiful daughter that ever the sun shone upon, the very finest drop-of-blood that ever trod on ground." ("Drop of blood", boinne, is a woman's name.) "I shall send fo rher and Cumal shall marry her and then, we may find our means to slay him here."
Between them, the two kings planned the marriage. The King of Lochlann sent for his daughter, the King of Ireland wrote a treacherous letter to Cumal, inviting him to come from Alba to Eire for a feast. Cumal came and there was feasting and joy.
When Cumal saw the daughter of the King of Lochlann, he fell heavy in love with her. Because Cumal was a grand, tall, handsome, stately man, the king's daughter fell in love with him as well. Therefore, both rejoiced when the Lochlannaich king suggested that they wed and they were married that very hour on the spot. The entire company put the bridal couple to bed, taking them through seven doors and seven rooms, each successive one inside the other, there, in the seventh room, they left the happy pair, leaving and locking each of the seven doors behind them as they left.
But Arc Dubh was hidden inside the innermost room, in accordance to the plan made between the two kings.
Cumal laid his sword on the board by the bedside. When all was still, the black traitor crept out from under the floor wherein he was hiding. He took up Mac a Luinn and laid it upon Cumal's neck where he lay sleeping. The weight of the sword, that never had to cut twice, took off the hero's head.
His bride was not aware of her father's treachery or the murder of her husband. When she awoke in the morning, she found him dead in her arms. She cried "Murder!" and the traitor, Arc Dubh, cried "Murder!" and the company opened all seven doors and came in to find Cumal dead and his new wife lamenting and beating her palms, full of sorrow and woe and heartbreak.
But the traitor took Mac a Luinn away with him to Eas Ruadh and there he stayed among the king's fishermen.