Krishisagar manthan( Krishi means milk, sagar means ocean and Manthan means Churning) Krishisagar manthan is also known as Samundar Manthan or Sagar Manthan.
Long long ago Indra The king of Devatas was ridding on his Elephant. Rishi Durvasa ( a sage) saw Indra and he offered him a garland. Indra accepted the garland with a smile and placed it on the trunk of his elephant. The elephant got irritated by the scent of he garland and he threw it on the ground seeing this the enraged sage Durvasa Cursed Indra and all Devatas to forget their Divya Shakties( Divya means Devin ans Shakties means Powers). In such a condition the Daanavas ( rivals of Devatas ) attacked the Havens and conquered the whole universe . The helpless Devatas went to Lord Vishnu for help. Vishnu advised them for Sagar Manthan .
The Sagar Manthan was a typical process.It was suppose to be done by both Devtaas and Danavaas,So Devtaas came into alliance with Asuraas (danavaas) for Sagarmanthan and share the emerged blessings. Amrit, the nectar of immortality (raised from Manthan) was the matter of concern for both Devataas and Danavaas. .
Vishnu secretly promised Devtaas to prevent Danavaas from drinking Amrit.
For churning Mount Mandarachala was used as a rod. Vasuki, the king of serpents, readly became the churning rope.
The mountain Mandarachala was heavy and it began to sink in the ocean, it began to sink.
Vishnu took the Avatar of Khurma ( Khurma is a sanskrit word for a turtle ) supported the mountain on his back.
In Samudra Manthan ,Halahal (a dangerous poison) emerged from the churning of cormic Ocean. The Halahal was so toxic that even a drop of it can distroy the earth. The Gods prayed Shiva for help ,on the advice of Vishnu. to protect the universe Shiva drank the poison but did not Swallow it. because of the poison strucked into Shiva’s throat the color of his neck turned blue that is why he is also called.Neelkhanta.
the 14 kinds of Gems which emerged out of cosmic ocean were divided among devtaas and danavas.
Lakshmi the Goddess of Wealth -also emerged out of cosmic ocean and she was accepted by Vishnu as his wife.
The Apsaras beautiful divine nymphs were taken by Devtaas(Gods).,
Varuni goddess of alcohol was taken by the danavaas.
Airavata and sveral other elephants, taken by Indra, the king of Devtaas and Uchhaishravas ,the seven headed horse was taken by danavas.
while Samundar Manthan Dhanvantri (the heavenly Physician ) emerged out of the ocean with the pot full of Amrit . seeing the Nector Pot Devtaas and Danvaas started fighting . Garuda took the divine pot and flew to Vishnu.
Vishnu took the Avatar of Mohini. Mohini was a lady with divine beauty . Mohini came with the Amrit pot to the battle place and she suggested devtaas and danavaas to sit in different row, so that she can distribute Amrit among them all the Devtaas and Daanavas agreed. Mohini Cleverly distributed The Amrit only among the Devtaas . One Danav Rahuketu sat in the row of devtaar and Drank the Amrit. The Sun God and the Moon God noticed him and they informed Mohini by that time Rahuketu already drank the Amrita mohini Cutt off his head with Sudarshan Chakra . Rahuketu could not die but his head was seprated from his body , his head came to be called as Rahu and his body Ketu.
Know the Devtaas were immortal so they fought for their Heavens from Danvaas.Once again Vishnu protected the world.