There was a king named Kalmashapada, a descendant of Ikshvaku, who was famed for his learning. While traveling in a forest, he encountered an ascetic, while walking on a narrow path. The path would admit just one.
An argument ensued regarding the right of way, each maintaining that the other ought to yield. Inflamed with rage, not stopping to consider his actions, the King struck out at the ascetic with his horse-whip. Angered, the Rishi cursed the King to become a flesh-eating Rakshasa.
While these exchanges were going on, the sage Vishwamitra, came that way. He recognized both the King and the ascetic, for the ascetic was none other than Shakti, the eldest of sage Vasishta's hundred sons.
Now, regarding the King, both Vishwamitra and Vasishta had wanted to make him their disciple, but neither had succeeded till this point. Vishwamitra seized this opportunity to revenge himself on both the king and Vasishta.
Vishwamitra concealed himself by his yogic powers and made the curse of Shakti take effect immediately. By his yogic power, he caused a Rakshasa spirit named Kinkara to enter the body of the King.
The King, under the influence of this Rakshasa, turned back to his palace. On the way home, he met a Brahmana, who begged him for food. Seemingly disregarding the beggar, the King returned to his palace. Once in his palace, he ordered his chief cook to prepare a meal of human flesh mixed with rice and feed it to the Brahmana whom he had met in the forest.
When the food was offered to that Brahmana, by his spiritual sight, he saw at once that the food was unholy. In his wrath, he cursed the king saying, "Since Kalmashapada has caused unholy food, made from human flesh to be fed to me, he shall develop a hunger for such unclean food. He shall be turned into a human flesh-eating Rakshasa!"
Thus reinforced, the curse became very strong. Impelled by the workings of fate, Kalmashapada once again returned to the forest where he had been cursed first. When he encountered Shakti again, he commenced his career as a human-eating monster by devouring the Rishi.
Vishwamitra, who was really the force behind the Rakshasa, then caused the King to hunt the other sons of Vasishta. One by one, Kalmashapada devoured them all.
When Vasishta came to know that all his sons were dead, by the stratagem employed by Vishwamitra, he became consumed by grief. If he had wished, he could have destroyed Vishwamitra utterly, but he had made a vow of peace, never to hurt any creature by his yogic powers.
Feeling that his life had lost all meaning with the utter destruction of his sons, he tried to commit suicide by various means. He was unsuccessful in this endeavor, for the ocean would not allow him to drown, nor did fire burn him. He tried to drown in a river named Haimavati, but the river, recognizing him as a Brahmana of great merit, fled in a hundred directions to avoid him. It is known to this day as Saptadaru (of a thousand streams). Thwarted in his attempt to commit suicide, he began wandering all over the world.
At last, he returned to his hermitage. As he approached his abode, he heard a young voice reciting the Vedas. When he entered his hut, he found that only his daughter-in-law Adrisyanti was there.
The mystery of the anonymous voice was solved, when the Rishi divined by his yogic power that it was the unborn child in her womb that had been reciting the Vedas! Glad that there was at least someone to propagate his race, the Rishi became consoled.
Much later, the Rishi saw the Rakshasa Kalmashapada in the forest. When the demon saw the sage, he tried to attack him with the intent of eating him. By his Yogic powers, the sage destroyed the Rakshasa spirit Kinkara and freed the King from his curse. The King, restored to his original form, fell at the feet of the sage and begged his forgiveness for his transgressions.
Vasishta said, "I have already forgiven you, for I know that your actions were the direct result of the curse. Go to your kingdom and rule justly, and never insult Brahmanas any more."