Once upon a time, there was a king in Erin who had thirteen sons. As they grew up, he taught them everything suitable for their status, from learning to various skills.

One day, while hunting, the king saw a swan swimming in a lake with thirteen little ones. The swan kept pushing away the thirteenth and wouldn't let it near the others. The king was puzzled and, upon returning home, called his old blind sage and said, "I saw something strange today—a swan with thirteen cygnets, and it kept driving away the thirteenth. Why would a mother reject one of her young?"

The sage replied, "Any creature, whether animal or human, with thirteen young should let the thirteenth go to find its own fate, so the will of Heaven can act on it without affecting the others. You have thirteen sons, and you must give the thirteenth to the Diachbha."

"So, I must give up my thirteenth son to the Diachbha?" asked the king.