It is to be known that on the night of the Feast of Samhain the doors separating this world and the next one are opened, and the inhabitants of either world can leave their respective spheres and appear in the world of the other beings.
Now there was a grandson to the Dagda Mor, the Lord of the Underworld, and he was named Aillen mac Midna, out of Shi' Finnachy, and this Aillen bore an implacable enmity to Tara and the Ard-Ri'.
As well as being monarch of Ireland her High King was chief of the people learned in magic, and it is possible that at some time Conn had adventured into Tir na n-Og, the Land of the Young, and had done some deed or misdeed in Aillen's lordship or in his family. It must have been an ill deed in truth, for it was in a very rage of revenge that Aillen came yearly at the permitted time to ravage Tara.
Nine times he had come on this mission of revenge, but it is not to be supposed that he could actually destroy the holy city: the Ard-Ri' and magicians could prevent that, but he could yet do a damage so considerable that it was worth Conn's while to take special extra precautions against him, including the precaution of chance.
Therefore, when the feast was over and the banquet had commenced, the Hundred Fighter stood from his throne and looked over his assembled people.
The Chain of Silence was shaken by the attendant whose duty and honour was the Silver Chain, and at that delicate chime the halt went silent, and a general wonder ensued as to what matter the High King would submit to his people.
"Friends and heroes," said Conn, "Aillen, the son of Midna, will come to-night from Slieve Fuaid with occult, terrible fire against our city. Is there among you one who loves Tara and the king, and who will undertake our defence against that being?"
He spoke in silence, and when he had finished he listened to the same silence, but it was now deep, ominous, agonized. Each man glanced uneasily on his neighbour and then stared at his wine-cup or his fingers. The hearts of young men went hot for a gallant moment and were chilled in the succeeding one, for they had all heard of Aillen out of Shl Finnachy in the north. The lesser gentlemen looked under their brows at the greater champions, and these peered furtively at the greatest of all. Art og mac Morna of the Hard Strokes fell to biting his fingers, Cona'n the Swearer and Garra mac Morna grumbled irritably to each other and at their neighbours, even Caelte, the son of Rona'n, looked down into his own lap, and Goll Mor sipped at his wine without any twinkle in his eye. A horrid embarrassment came into the great hall, and as the High King stood in that palpitating silence his noble face changed from kindly to grave and from that to a terrible sternness. In another moment, to the undying shame of every person present, he would have been compelled to lift his own challenge and declare himself the champion of Tara for that night, but the shame that was on the faces of his people would remain in the heart of their king. Goll's merry mind would help him to forget, but even his heart would be wrung by a memory that he would not dare to face. It was at that terrible moment that Fionn stood up.
"What," said he, "will be given to the man who undertakes this defence?"
"All that can be rightly asked will be royally bestowed," was the king's answer.
"Who are the sureties?" said Fionn.
"The kings of Ireland, and Red Cith with his magicians."
"I will undertake the defence," said Fionn. And on that, the kings and magicians who were present bound themselves to the fulfilment of the bargain.
Fionn marched from the banqueting hall, and as he went, all who were present of nobles and retainers and servants acclaimed him and wished him luck. But in their hearts they were bidding him good-bye, for all were assured that the lad was marching to a death so unescapeable that he might already be counted as a dead man.
It is likely that Fionn looked for help to the people of the Shi' themselves, for, through his mother, he belonged to the tribes of Dana, although, on the father's side, his blood was well compounded with mortal clay. It may be, too, that he knew how events would turn, for he had eaten the Salmon of Knowledge. Yet it is not recorded that on this occasion he invoked any magical art as he did on other adventures.
Fionn's way of discovering whatever was happening and hidden was always the same and is many times referred to. A shallow, oblong dish of pure, pale gold was brought to him. This dish was filled with clear water. Then Fionn would bend his head and stare into the water, and as he stared he would place his thumb in his mouth under his "Tooth of Knowledge," his "wisdom tooth."
Knowledge, may it be said, is higher than magic and is more to be sought. It is quite possible to see what is happening and yet not know what is forward, for while seeing is believing it does not follow that either seeing or believing is knowing. Many a person can see a thing and believe a thing and know just as little about it as the person who does neither. But Fionn would see and know, or he would under-stand a decent ratio of his visions. That he was versed in magic is true, for he was ever known as the Knowledgeable man, and later he had two magicians in his household named Dirim and mac-Reith to do the rough work of knowledge for their busy master.
It was not from the Shi', however, that assistance came to Fionn.