Mahabharata I.178 describes the story of Shakti.
The fight between Sage Vasistha and Kausika, later known as Vishwamitra is well known and was described in the Bala Kanda of Ramayana and in Mahabharata. Shakti was the eldest son of Sage Vasistha. Adrisyanti was the wife of Shakti.
One day the sons of Sage Vasistha had a tussle with a king called Pravriddha, later known as Kalmashapada, with respect to right of way on a narrow path. As the King struck out at the Shakti with his horse-whip, the Rishi cursed the King to become a flesh-eating Rakshasa.
Taking the situation to his advantage Kausika, aka Viswamitra caused a Rakshasa spirit named Kinkara to enter the body of the King. Later when Kalmashapada encountered Shakti again, he commenced his career as a human-eating monster by devouring the Rishi. One by one, Kalmashapada devoured all 100 sons of Sage Vasistha.
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.
The son born to Adrisyanti was named Parasara. He learned the scriptures under the guidance of his grandfather. Parasara is the father of Veda-Vyasa, who composed the Mahabharata.