Once the eight Vasus (Ashtavasus) visited Vashishta's ashram accompanied by their wives. One of them, Prabhasa's wife got extremely fond of the Kamadhenu cow. (Kamadhenu, also known as Surabhi, is the mother of all cows. She is miraculous and provides her owner whatever he desires for.) So the wife asked Prabhasa to steal the cow and he then stole the cow with an assistance provided by the rest of seven vasus. Hence they were all cursed by Vashishta to be born as mortals. The Vasus appealed for mercy and Vashishta moderated the curse of the first seven Vasus (Who had assisted Prabhasa) that they would be liberated from their mortal birth as soon as they were born. However, Prabhasa being the protagonist of the theft was cursed to live a long life on the earth filled with loneliness, pain, sadness and frustrations. The curse was softened to an extent that he would be one of the most illustrious men of his age.
The eight Vasus didn't desire to be born to an ordinary woman hence they went to Mother Ganga and requested her to give birth to them and drown the first seven soon after so that they can go back to heaven. Hence the first seven sons of Ganga and Shantanu were drowned in order to break their curse and Prabhasa was the one born as Devavrata or Gangeya, later known as Bhishma. Shantanu had agreed to Ganga's condition to never question any of her actions and this is the reason he remained quite when the first seven were drowned.
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Now coming to Duryodhana and Shakuni, these two were incarnations of Kali and Dwapar demons. Kali's companion was Dwapar and their incarnations were related on the earth too. Mama Bhanja 😆 Not lot info is available about Dwapar. I'm sharing some information from various sites.
"In Hinduism, Kali (IAST: kli; Tamil: ; Devngari: ; from a root kad "suffer, grieve, hurt; confound, confuse") is the reigning lord of Kali Yuga and archenemy of Kalki, the 10th and final Avatar of the Hindu god Vishnu. In the Kalki Purana, he is portrayed as a demon and the source of all evil. In the Mahabharata, he was a gandharva who possessed Nala, forcing him to lose his Kingdom in a game of dice to his brother Pushkara. His most famous incarnation is the Kaurava King Duryodhana. Kali is similar to the demon Kroniand his incarnation Kaliyan of Ayyavazhi mythology."
"Kali was later incarnated as king Duryodhana, eldest of the one hundred Kaurava brothers. His companion Dvapara became his uncle Sakuni. The day Duryodhana was born, he unleashed a donkey-like scream which the donkeys outside the home replied to. Despite the advise from Vidura to discard the evil baby, Duryodhana's father Dhritarashtra kept the child because demons had received a boon from Shiva that the future king would be invincible."
http://en.wikipedia.org/wiki/Kali_(demon)
One more interesting post -
http://www.manuscrypts.com/myth/2010/05/14/amshaavatar/
Amshaavatar
Posted on May 14, 2010As evil in the world increases, Vishnu and other devas incarnate in full or in part to rid the world of them. The full avatars are Purnaavatar and the part avatars are Amshaavatar. The Mahabharata Adiparva Chapters 54-64 has a complete list. Below is a list from the Shrimahadevibhagavata 4th Skanda ()Deva - Avatar)
Kashyapa - Vasudeva
Adisesha - Balabhadra
Narayanarshi - SriKrishna
Yamadharma - Yudhishtira
Ashwinis - Nakula, Sahadeva
Dharma - Vidura
Shiva - Ashwathama
Gandharvaraja - Devaka
Ashtavasu (one of the ashtavasus) - Bhishma
Marudganam - Kripa, Kritavarma
Aditi - Devaki
Nararshi - Arjun
Vayu - Bhima
Surya - Karna
Varuna - Shantanu
Marut - Virat
Hansa - Dhritarashtra
Brihaspati - Drona
Dwapara - Shakuni
Pavaka - Dhrishtadyumna
Kali - Duryodhana
Rakshasa - Shikhandi
Varuna - Drupada
Dhriti - Madri
Viprachitti - Jarasandha
Hayagriva - Keshi
Bashkala - Bhagadatta
Lamba - Pralamba
Sanalkumara - Pradyumna
Lakshmi - Panchali
Sidhi - Kunti
Mati - Gandhari
Jaya - Hiranyaksha
Vijaya - Hiranyakashipu
Hiranyaksha - Ravan
Hiranyakashipu - Kumbhakarna
Ravan - Sishupala
Kumbhakarna - Dantavaktra
Prahlad - Salya
Kalanemi - Kamsa
Anuhlada - Dhrishtaketu
Khara - Dhenuka
As evil in the world increases, Vishnu and other devas incarnate in full or in part to rid the world of them. The full avatars are Purnaavatar and the part avatars are Amshaavatar. The Mahabharata Adiparva Chapters 54-64 has a complete list. Below is a list from the Shrimahadevibhagavata 4th Skanda ()Deva - Avatar)
Kashyapa - Vasudeva
Adisesha - Balabhadra
Narayanarshi - SriKrishna
Yamadharma - Yudhishtira
Ashwinis - Nakula, Sahadeva
Dharma - Vidura
Shiva - Ashwathama
Gandharvaraja - Devaka
Ashtavasu (one of the ashtavasus) - Bhishma
Marudganam - Kripa, Kritavarma
Aditi - Devaki
Nararshi - Arjun
Vayu - Bhima
Surya - Karna
Varuna - Shantanu
Marut - Virat
Hansa - Dhritarashtra
Brihaspati - Drona
Dwapara - Shakuni
Pavaka - Dhrishtadyumna
Kali - Duryodhana
Rakshasa - Shikhandi
Varuna - Drupada
Dhriti - Madri
Viprachitti - Jarasandha
Hayagriva - Keshi
Bashkala - Bhagadatta
Lamba - Pralamba
Sanalkumara - Pradyumna
Lakshmi - Panchali
Sidhi - Kunti
Mati - Gandhari
Jaya - Hiranyaksha
Vijaya - Hiranyakashipu
Hiranyaksha - Ravan
Hiranyakashipu - Kumbhakarna
Ravan - Sishupala
Kumbhakarna - Dantavaktra
Prahlad - Salya
Kalanemi - Kamsa
Anuhlada - Dhrishtaketu
Khara - Dhenuka