The churning of the milky ocean for amrita is one of the popular stories from the Puranas. Rishi Sauti narrated it in the Adi Parva of the Mahabharata.
A long time ago, the devas met on the mystical Meru Mountain. The repeated wars with asuras were affecting them. Many devas were being injured or killed. Though Mount Meru had many magical herbs, they wanted a better solution, something like a one-time drink to make sure the injured healed automatically and made them immortal. After the meeting, they went to Brahma Deva with their request. He took them to Narayana who advised that they churn the milky ocean to obtain amrita. Drinking the nectar of immortality would solve their problems.
Narayana also advised the devas to convince the asuras to join them in the process, as it was laborious. He assured them that he would ensure the amrita wouldn’t be misused. The devas first went to the milky ocean and asked permission to churn it. With advice from Narayana, they requested Sesha (Ananta) to uproot and bring the Mandara Mountain for churning. To prevent the setup from sinking, they asked Akupara, the king of tortoises for help. The majestic tortoise carried the weight of the mountain on his back. Sesha wrapped around the mountain. The devas held his tail while the asuras held his head.
The churning began. Sesha hissed venom in the process, but the asuras could handle it. However, both sides were soon exhausted. Narayana used his powers to shower them with endless energy until they achieved the task. Slowly, different items began to rise from the milky ocean.
The moon arose, full, bright, and soothing. Goddess Lakshmi came out sitting in a lotus flower and married Narayana. The goddess of wine, Sura Deva, rose with a jar in hand. Then rose the white horse, Ucchaihshravas. A large red ruby named Kaustubha rose and adorned Narayana’s chest. Dhanvantri, the healer of the devas, rose from the milky ocean with a pot in hand. He brought the amrita to the surface. The asuras stole the pot from him.
As the devas were dejected, Narayani transformed into Mohini, a beautiful young woman. Mohini went in search of the asuras and tricked them into giving the pot of amrita to her. Back in heaven, Narayana distributed the amrita to the gods. However, a danava followed Mohini to heaven and licked a drop of the amrita by hiding behind some devas. Surya Deva and Soma Deva saw this and informed Narayana, who sliced the danava’s head with his chakra. However, the amrita made him immortal. The danava became two beings – Rahu, the head, and Ketu, the body. They swore revenge on the sun and moon and tried to swallow them whenever possible. However, they couldn’t hold on for long and would eventually release them.
In the next war with the asuras, the devas could win without casualties. They asked Narayana to keep the amrita safe. He helped Indra, the ruler of heaven, safeguard the amrita with various security measures.
The wish-fulling cow, Surabhi/ Kamadhenu, the parijata tree, Airavata, a four-tusked white elephant, and the deadliest poison, Kalakuta, also rose from the milky ocean during the churning. The cow was gifted to the rishis. Indra kept Airavata, and his wife, Sachi, asked for the parijata tree. The white-winged horse was gifted to Bali. Shiva collected the poison and held it in his throat to prevent the destruction of the world.