Shesha and Vasuki

Of the Naga sons of Kadaru, there was one called Shesha, who was pious and holy in character. He could not tolerate the ways of his brothers, so one day he left all and went off into the forests to meditate. His meditation was very austere.
Pleased with his devotion, Brahma appeared before him and wanted to know the reason for his prayers..
Shesha said - Garuda is a brother to me. All my brothers have become enemies of Garuda due to their own behavior. Brahma said your prayer has been selfless and you have not asked for anything for yourself. I bestow on you the weight of the world. On your hood, O thousand headed snake, you will balance the earth.

Back among the Nagas, they were all scared. They had been cursed by their mother to die in the snake sacrifice of Janmajeya. Would there be a way out?
The wisest among them, Vasuki, asked his brothers not to worry. He said when their mother had cursed them, the Gods had also decided on the way out.
Jaratkaru, the rishi would marry Jaratkari, sister of Vasuki. They would have a son called Astika, who would put an end to the sacrifice. 

Jaratkaru

Jaratkaru was a sage engaged in sever penances. Lost in meditation, he would roam around the forests, unclothed.
One day he saw a huge pit in the forest. A root of a tree was covering the pit, and from the root were hanging some sages.
Jaratkaru asked them who they were and why they hanging like this. He also told them that a forest rat was slowly gnawing at the root and after sometime the root would break, and all would fall in the pit.

The sages said they were his ancestors. They would be relieved if Jaratkaru married and had a family. Only then they would salvation. They asked Jatarkaru to get married and then have a son who would be able to carry the line forward.

Jatarkaru said he had forsaken everything and was roaming around, naked. Who would agree to marry such a person.

He started roaming everywhere, this time in search of a wife. In the meantime, the spies of Vasuki, the serpent, told him that Jaratkaru was looking for a wife. Vasuki went to Jaratkaru and asked to marry his sister. His sister was called Jaratkri. Vasuki gave away his sister to Jaratkaru.