Long time ago , eight Vasus visited the ashram of Maharishi Vashistha . Vashistha honored them all and provided all the amenities. All the Vasus were very happy with this honor of Maharishi Vasistha. While they were leaving, wife of the Vasu named, Dhau, saw a Kamadhenu cow at Maharishi vashistha’s ashram and asked her husband about her. Upon hearing his wife,the Vasu named Dhau said, “dear after drinking the milk of this Kamdhenu cow,a man will never get any disease and does not even get old age.” Hearing these words, vasu dhau’s wife said that you take this cow with us for a few days. I have a friend in Prithvi Lok, I want to give her milk of this cow ,so that she can remain a young girl forever and get rid of all diseases.
Listening to his wife, Vasu named Dhau stole Kamadhenu after discussing with other Vasus, insulting Maharishi Vasistha. When Maharishi Vasistha came back to his ashram and could not find Kamadhenu there, he searched around for Kamadhenu . He went to the forest, went to the mountains, even after searching at various places he could not find Kamadhenu. Then Maharishi Vasistha meditated and through his yogic power came to know that the Vasu named Dhau stole his Kamdhenu cow and also insulted him and the rest of the Vasus have also supported him, knowing that Maharishi Vasistha cursed the Vasus that they would have to be born in as humans.
When all the Vasus came to know that Vashistha cursed them, they became very frightened and came to Vasishta’s ashram to ask for forgiveness. With folded hands in front of him, they confessed their mistake and asked the path to be free from the curse. Then Vashistha said that all of you must definitely take birth in human form, the crime of all of you is less, that is why you will get freedom from human form , immediately after taking birth. But this vasu named Dhau will have to stay on earth for some time taking human form. Hearing the words of Vashistha, all the Vasus came out of the ashram and found Ganga ji there, who came to earth taking t human form due to the curse of Brahma.
All the Vasus prayed to Ganga ji to help them in getting free from the curse of Maharishi vashistha. Ganga accepted this and agreed to become their mother in human form. Later Goddess Ganga married Shanranu , king of Hastinapur Kingdom. While marrying, Ganga put a condition in front of King Shantanu that he should never ask her reasons for her activities and should never interrupt in her activities . The day Shantanu fails to fulfill this condition she will leave the king and will not be her wife any longer. Shantanu accepted all the terms of Goddess Ganga and married her. After some time, Goddess Ganga had a son, but Ganga drowned that son in the waters of Ganges. In this way, seven sons were drowned in water of river ganga, but Shantanu did not say anything.
When the eighth son was born, Maharaj Shantanu could not stop himself and he prayed to Ganga not to kill this child. Seeing her condition broken, Ganga started saying, Maharaj, I also do not want to kill this child, you know the reality of me, I am Goddess Ganga, who came here from the curse of Brahma. The reason I killed the sons is because, all of them are actually cursed by Vasishtha. The eighth son is also a cursed Vasu named Dhau. He will remain as your son for some time and stay on the earth. Right now I am taking him with me. I will give him to you when the right time comes, through a proper master I will make him master of all disciplines and capable enough to become your successor
This eighth son of Ganga was named Devrath. When he turned 16, Ganga handed him over to Maharaj Shantanu. The same Devavratha later was called Bhishma because of his vows. In this way, the Vasu named Dhau was born as son of Shantanu in the world.