Rahu was a demon ,son of Simhika He took penace or many years to obtain Amrut [nectar] .Bramha gave him the boon to obtain thus. Amrut gives one immortality.
Once Sage Durvasa was giving a discourse on Lord’s supremacy .Indra was passing by . So the rishis listening to the discourse ,made a garland and offered it to The Indra as a mark of respect . Indra out of haughtiness ,kept the garland on the Elephant [kumbhasthala] ,As elephant shook its head the garland fell down . Seeing the disrespect and carelessness of Indra , Durvasa cursed him to loose his opulence.
Immediately Demons took over the kingdom of swarga and Devatas lost their charm. All of them went to Kshirsagar and requested Narayana to end their plight . Narayana advised them ,since their time is presently not favourable ,they must make mends with Demons and get ready for great churning of the ocean together and share everything that comes out of it.
Devatas and Danavas agreed . To churn they chose the Mandar parvat . But none could lift it . So God effortlessly plucked it and summoned Garuda[Anantha] and he took it to the place of action . But again both Devatas and demons together tried to grip the Mandar parvat , but it slipped and fell on the earth and many got disabled and crippled under it . Again God with his left hand plucked it and placed it on Garuda ,and also cured those crippled . Garuda with his wings held it for Churning but ocean could not be churned and it slipped to the bottom of the ocean .
Lord Narayana then took the form of the Tortoise [Kachchap] Kurma , held the mountain Mandar on its back and brought it above the ocean and it stayed still supported on the Kurma. Narayana entered the mandar , devatas and demons to renew their strengths to churn the ocean with the Vasuki the snake offering the services as rope to churn the ocean with the golden Mandar.
Demons preferred to hold the face of the Vasuki as tail was inauspicious. But as the churning would commence ,Vasuki breathed heavily and dispelled poison and fire though its mouth causing distress to Demons and Devatas at the tail were at ease. [Evil intentions bear always negative results and discomfort even if good choices are made , here though a face is more auspicious than tail ,it turned out to be uneasy option]
While churning the deadly poison of KaalKuut emerged . This was very embodiment of Kali . By the boon of Bramha ,it could not be even touched as it burnt heavily . Lord Vayu took it in his palm and in a Golden Vessel . He took a drop out of it , placed in his palm and rubbed it to reduce its potency , and then a small portion of it ,placed on the tongue of Shiva , Shiva before it could reach his throat , fell unconscious , his Neck turned blue [ He was thus called NEELKANTH] . Lord Vayu drank the whole vessel full of poison and nothing happened as he digested it.
As the drop was being given to Shiva , a few droplets fell on the earth , as it was Kali himself ,it spread rapidly and the poison assumed various forms of manifestations as .serpents , wolf, lions and other poisonous creatures and Poisonous tribes of maneaters and cannibals. Many weapons of poisonous nature also emanated from it .
Lord Narayana placed his gracious palm on the Shiva ,he got up ,but unable bear heat of the poison , He chose to stay in the cold snow regions of Kailash near Meru ,but still his pain and heat was unbearable ,thus he kept Ganga in his hairLocks [Ganagadhar] ,still he could not relieve severe headache ,thus he Kept the Moon on his head . Yet to save himself from pain and swelling ,he invited serpents round his neck to give soothing effect.
Churning continued a whole vessel of intoxicating Madya came up Demons accepted it ,and out came Uchhaishrava ,Bali kept it for himself . Then came Airavat ,it was given to Indra . Many Apsaras emanated they were all sent to Swarga . Then came Moon , Parijat , Kamadhenu and Kalpavraksha ,Kaustubha gem , Lord Narayana accepted it as a present from Bramha. Then came IndiraDevi as Mahalakshmi , She adored the chest of Narayana as Srivatsa.
Then finally Lord Dhanvantari came up with a kamandalu and a vessel full of nectar[Amrut] . Rakshasas snatched it from HIM . God allowed it to be snatched. as one must commit crime to be punished , Demons transgressed the rule that everything must be shared ,thus were entitled to be punished .Demons started fighting amongst each other as Narayani the beautiful form of the Lord appeared .
Demons struck by her beauty handed over the pot of nectar to her distribute . Devatas and Demons sat in opposite rows and Mohini , poured the nectar to Devatas and just water to Demons.Here God betrayed Demons to validate the dictum according to the Dharma
Dharmachhellam papajaneshu Dharma
Cheating evil people is akin to Dharma or One must not apply usual parameters of Dharma to sinners. one must pay back in the same terms as used by sinners.
But Rahu took his position between Sun and the Moon among the devatas . Lord put a drop in his palm [to honour the Bramha’s words] But when alerted by Sun and moon , he slit of the throat of the Demon with his Sudarshan chakra. But by then the Nectar had already entered his neck . Thus his both head and the Body [headless kabandh] became immortal . Head became one of the navagrahas and the body is still in the Shubodh Sagar . Aspiring for things undeserving always results in disaster . Though Rahu aspired for Amrut but failed to obtain grace of Lord Narayana ,he suffered .But vowed to avenge ,Thus time and again eclipses Sun and Moon ,who suffer at the hands of Rahu owing to his immortality . Though Rahu’s head was severed by Narayana , just a mere touch of Amrut entitled him to survive .[ what a greatness of Amrut the nectar !].