Amrita the nectar of immortality, was churned out of celestial ocean of milk (Kshherasagaram) by the gods and demons. The story of getting Amrita is described in ancient Hindu scriptures. The legend has great importance in both Saivite and Vaishnavite system of beliefs.
Once there was continuous war between gods and demons. The gods were afraid of loosing the battle. Vishnu suggested them to churn the ocean of milk to get Amrita. By drinking Amrita gods will be immortal. The Devas and Asuras (gods and demons) united their efforts to churn the ocean of milk in quest of Amrita. The gods were holding the tail of Vasuki while the demons were holding the side of the head of the serpent. Mandara Mountain was used as a churning stick and mythological serpent Vasuki was used as the churning rope.
Vishnu took the form of his incarnation, Koorma(tortoise) to hold the Mandara mountain in place so that it does not sink into the ocean.
Many precious things came out by churning the sea. Lakshmi, the Goddess of wealth emerged from the ocean and also the moon god Chandra. The divine elephant Airavata came out which was occupied by king of god, Indra. Then emerged the precious stone Kaustuva, which Lord Vishnu put on his chest. After these came the dreaded poison, Kalkoota, which was so toxic that all the creation was about to destroy. The gods and demons appealed to Lord Shiva to save them from the fumes that the poison yielded. Lord Shiva consumed the poison to save the world and retained it in his throat, which turned blue due to the effect of the poison. Henceforth Shiva is known as the name of `Neelakantha` or `Vishaapaharana Murthy`.
Devas and Asuras continued with the churning of ocean after their obstacles were overcome in quest of their ultimate goal, Amrita. Finally they got their reward when the divine physician Dhanvantari emerged out of the ocean with a pot bearing the celestial necter of immortality or Amrita.
A fierce tussle followed between the gods and the demons regarding who will have the share of Amrita. Again Vishnu took the responsibility of protecting the god. He took the guise of a beautiful charming lady named Mohini. Mohini persuaded the Asuras with her charm and served all the Amrita or ambrosia to the gods. Thus Lord Vishnu saved the world from the calamity that was about to take place if the Asuras had attained immortality by drinking Amrita.
Two of the Asuras were aware of the trick of Mohini. They took the guise of gods and got a dose of Amrita. Vishnu came to know the fact and when the Asuras had consumed only half the dose of Amrita, he proceeded to destroy them.
The Demons took the form of Rahu and Ketu respectively. Rahu had snake`s head and human body while Ketu had human head and snake`s body. Vishnu destroyed Rahu`s body but his head was saved due to consuming Amrita up to the throat. It is believed that Rahu is the cause of solar and lunar eclipse as he engulfs Sun and Moon because they could see him through his guise.
Another legend tells that serpent Vasuki consumed the Amrita against the wishes of sun and moon and his body was split into two demons, Rahu and Ketu and thus they became permanent enemy of sun and moon causing the eclipse.
