Once King Janamejaya requested sage Vaishampayana to narrate the story of Samudra manthan (churning of the ocean of milk). The Sage began narrating thus – 'O king! Long ago the gods and demons decided to obtain nectar (amrit) and other gems by churning the ocean. Mount Mandarachala (mount Meru) was used as churning rod and Vasuki, the king of serpents became the churning rope. On one side, the gods held the tail of Vasuki while demons held its head on the other side and began churning. However, during churning the mountain began to sink, Vishnu then incarnated in the form of a turtle, Kurma and supported the mountain on His back. When the churning commenced, an extremely potent poison halahal emerged, which distressed all of them. The gods and demons appealed to Shiva who swallowed the poison. The following fourteen treasures came out of the sea viz. Lakshmi, Kaustubha, Parijatak, Sura, Dhanvantari, Chandra, Rambha, Airavat, Nectar, Poison, a horse named Uchhaishrava, Kamdhenu, Shankh (conch) and Haridhanu.
While distributing the obtained treasures, there was a dispute between the gods and demons concerning nectar and sura. Vishnu, settled the dispute by saying that gods and demons should sit in separate rows someone would distribute nectar and sura; and that they should be contended with what they got. Vishnu then manifested as a beautiful lady, Mohini and started the distribution. The demons were distracted by Mohini's beauty while she distributed nectar amongst the gods and sura amongst demons. Rahu, a demon disguised himself as a god and received his portion of nectar (Amrit) which he quickly drank. Chandra (the moon god) informed Vishnu about it. Vishnu immediately decapitated Rahu with the Sudarshan chakra. His head went up in the sky while his remaining body travelled towards the western ocean. The gods and demons tried to destroy his body. Shiva tried to destroy Rahu's body with his trishul (trident).However, due to the nectar in his stomach, it couldn't be destroyed. Shiva then took Rahu's body on Mount Mhalasa and ejected the nectar from it. The same nectar started flowing into the sea. This stream is called Pravara River, which is a tributary to river Godavari. At the confluence of rivers Pravara and Godavari, Mohini, called as Mhalasa, is seen seated on Rahu's body.