Raajaa Nahush was the son of Aayu (the eldest of Pururavaa). He had a son named Yayaati. By sacrifices, Yagya, Tap Nahush got the sovereignty of the three worlds

According to one version, he became Indra at one time. As he became Indra, he thought that he could have Indraanee (Shachee - Indra's wife). He sent a message to Indraanee that since he has become Indra he was coming to her palace. Indraanee got worried but couldn't do anything. He was very happy that he could have Indranee. He asked some Rishi to carry his Paalakee to Indra's palace. 

He forgot everything in that happiness and he employed a thousand Rishi to carry his Paalakee. Sine Rishi were not used to this work they were moving slowly. But Nahush was in hurry so at one time he push4ed Agastya Muni with his foot and said "Sarp, Sarp" mean "hurry, hurry". Now "Sarp" means snake also, so Agastya Muni said - "You are calling us "Sarp, Sarp" You yourself become Sarp and fall on Prithvi." The same moment he fell down on Earth and became a serpent.

On asking forgiveness from Muni, Muni said - "You will be released from this Shaap when you will catch the mightiest person on the earth and Yudhishthir will answer your all spiritual questions."