King Amshumat was the son of Asamanja and grandson of King Sagara and his story is found in the Bala Kanda of Ramayana. Once, King Sagara decided to perform a great horse sacrifice. Amshumat, who was the greatest archer of his time, tended to the horse. In horse sacrifice, a horse is left free to roam around. Kings who allow the horse unimpeded access agree to pay tribute to the king who owns the horse and who is undertaking the horse sacrifice. Those unwilling to do so, seize the house and a battle ensues. Amshumat followed the horse and defeated all the kings who captured the horse.
Amshumat returned successfully with the horse but on the night of the great sacrifice, Indra, the king of Devas or demigods, took the form of a female demon and stole the horse.
King Sagara ordered his 60,000 sons to find and bring back the horse. They spotted the horse near the ashram of Kapila. The sons thought that Sage Kapila had stolen the horse and accused him of theft. The sons rushed towards Sage Kapila to kill him. Hearing their words and action, the immeasurable and great souled Kapila was overcome by great anger and uttered the sound of humkara. The sound reduced the 60,000 sons of Sagara into ashes.
When Sagara saw that his sons had been gone for a long time, he asked Amshumat, his grandson, to find them. Amshumat reached the spots of dishagajas – the four elephants believe to hold up the four directions of the earth. They blessed him with success in his endeavor.
Soon Amshumat reached the spot where his father and other sons of King Sagara had been reduced to ashes. He was overcome by grief on witnessing the mounts of ashes. He also spotted the sacrificial horse wandering around.
He wanted to perform shradh and tarpan to the departed souls but he was unable to find water for the purpose. In the barren land, he saw Garuda, who advised him that the soul of his father and others will attain moksha when Ganga flows through the barren land. Garuda asked Amshumat to take back the horse and complete the sacrifice.
Amshumat obeyed the orders of Garuda and took back the horse to King Sagara who completed the sacrifice. However, the king could not make up his mind about how Ganga was to be brought to earth to give moksha to his children.
Amshumat became the king and he was extremely devoted to dharma. He had a great son named Dileepa.
Amshumat too was unable to find a way to bring Ganga from heaven to earth. After ruling for several years, he handed over his kingdom to Dileepa and went to the Himalayas. He performed intense austerities for several thousand years and finally attained moksha.