Amsumana was the grandson of king Sagara of the solar dynasty, who ruler over Ayodhya. He had a grandson named Asmanja. Since Asamanja used to take delight in throwing the children of the subjects of the kingdom into Sarayu River even as a crown prince, Sagara banished him from the kingdom at the request of the people. Son of Asmanja, Amsumana, adept in all branches of learning and valiant, was crowned king.
King Sagara once performed a horse-ritual. He could not complete it as Indra, the king of gods, out of fear of losing position, stole the horse and tied it up in the nether world close to a place where Sage Kapila was engaged in Tapas.
In order to retrieve the sacrificial horse, Sagara sent his 60,000 sons. They were all reduced to ashes by the sage when they insulted him.
Sagara was worried over the non-completion of his ritual as it spelt disaster for him. He sent Amsumana to find out what had happened to his 60,000 sons and the horse. Amsumana went to the netherworld and by his winsome ways, valor and wisdom, was able to locate the horse and bring it back, enabling king Sagara to complete the yajna. But he could not ensure the salvation of his uncles who had been reduced to ashes, as their last rites had not been performed. After ruling the kingdom righteously for many years, Amsumana left for the Himalayas to undertake penance. His son Bhagiratha became the king. Bhagiratha finally performed the funeral rites of the sons of Sagara by making the waters of the celestial Ganga flow over the ashes of the 60,000 dead princes.