Trishanku and Sagar

Paraashar Jee said - "Maandhaataa had the son named Ambareesh.

Ambareesh's son was Yuvanaashwa
Yuvanaashwa's son was Haareet
Haareet's sons were Angiraa Gotra Haareet Gan

In earlier times times there lived 60 million (6 Crore) Mauneya named Gandharv in Rasaatal. They took away all principal gems and rights from all Naag families. So Naag went to Vishnu Bhagavaan and told about their plight. Vishnu said - "Yuvanaashwa's son Maandhaataa has a son named Purukuts. I will enter his body and kill all those Gandharv." They came back to Purukuts and inspired his wife Narmadaa (who was their sister too) to bring him to Rasaatal. She brought him there. There Vishnu entered his body and Purukuts killed all Gandharv and came back to his own city. At the time of departing, Naag gave a Var to their sister Narmadaa that whoever will take your name, he will not have any fear from snakes' poison. There is a Shlok about this -

Narmadaayai Namah, Praatar-narmadaayai Namo Nishi |
Namo-astu Narmadey Tubhyam Traahi Maam Vish Sarpatah ||  [p 243]
[Vishnu Puraan, 4/3/13]
read this Shlok in Sanskrit here - Mantra for Snake Bite

If somebody goes around pronouncing this Shlok in dark, snake never bites him.

Purukuts' son was Trasdasyu (from Narmadaa)
Trasdasyu's son was Anranya - he was killed by Raavan at the time of his victory tour
Anranya's son was Prishdashwa
Prishdashwa's son was Haryaashwa
Haryaashwa's son was Hast
Hast's son was Sumanaa
Sumanaa's son Tridhanvaa
Tridhanvaa's son was Trayyaaruni
Trayyaaruni's son was Satyavrat who was later known as Trishanku.

Trishanku had become Chaandaal. Once there was no rains for 12 years . At that time to sustain Vishwaamitra's family and to be free from his Chaandaal behavior, he used to hang a deer meat on a banyan tree on the banks of Gangaa River. Pleased with this, Vishwaamitra sent him to Swarg with his mortal body.

Trishanku's son was Harishchandra
Harishchandra's son was Rohitaashwa
Rohitaashwa's son was Harit
Harit's son was Chanchu
Chanchu's sons were Vijaya and Vasudev
Vijaya's son was Ruruk
Ruruk's son was Vrik
Vrik's son was Baahu

Haihaya and Taaljangh Kshatriya defeated Baahu so he went to forest with his pregnant queen. Raajaa's another wife gave her poison out of jealousy. Because of this poison her fetus stayed in her womb for seven years. After that Baahu died near the Aashram of Aurv Muni. His pregnant queen decided to become Satee with him, but Aurv Muni stopped her doing so, saying - "You have the Swaamee (Lord) of the whole Prithvi, a Chakravartee king, in your womb, therefore you should not think of doing this." Aurv Muni brought her to his Aashram where her son was born with that poison.

Because of being born with that poison (Gar), he was called Sa-Gar = Sagar = with poison. They lived there only for some time. Maharshi Aurv taught him Ved, Shaastra and Bhaargav named Aagneya weapons. When Sagar had grown, he asked his mother - "Why do we live here in this forest? And where is my father?" Then his mother told him everything. He destroyed all Haihaya and Taaljangh families. Other families jointly went to Sagar's Kul Guru Vashishth Jee to complain about this. Vashishth Jee said to Sagar - "Son, Why are you killing these living dead people?" Sagar obeyed him and stopped their killing. He changed their guises - he shaved Yavan's heads, half-shaved Shak's heads, asked Paarad to wear long hair etc.