When young, Niall and his four step-brothers, Brian, Fiachra, Ailill and Fergus were given weapons by a smith and sent out hunting to prove their arms.
After losing their way in a forest, the five youths lit a fire to cook the game they had killed, and Fergus went in search of drinking water.
He came to a well guarded by a monstrous Black-Hag who would grant him the use of the well only on condition he gave her a kiss.
Fergus fled screaming…
Each of his three brothers in turn undertook the same errand but only Fiachra deigned to give the Black-Hag the merest brush of a kiss for which he was promised, in return, the merest of contacts with Tara.
Faced with the self same challenge, Niall kissed and embraced the Black-Hag who, when he looked again, had changed into the most beautiful woman in the world.
“Who are you?” asked Niall.
“I am Sovereignty,” replied the woman, “and your seed shall be over every clan.”