The story of Rahu & Ketu

There were constant wars between the Devas and the Asuras. The Asuras always won, due to their guru Shukracharya's Mritha Sanjivini power, which the guru had obtained as a fruit of his penance, propitiating Lord Shiva. The power of this Mritha Sanjivini is, it can resurrect the dead, back to life. In this way, each Asura who was killed in the war by the Devas, was brought back to life by this manthra. Thus, this ensured constant defeat for the Devas and the Asuras continued to rule the heavens.

Lord Indra, the king of the Devas, went to the abode of Lord Vishnu, with all the Devas, and, prayed to the Lord, asking him for a solution. Lord Vishnu, asked the Devas to churn the milk ocean (Saagar Manthan), and, consume the amrith that emerges out of this ocean. For this, they would need to take the help of the Asuras.

Lord Indra, went to heavens, and, sought for peace, and, said that henceforth, the Devas and Asuras should never wage war against each other, and, told them the need for churning the milk ocean and the power of  consuming the Amrith. The Asuras, agreed, and inside their evil minds, they thought, once the churning is over and the Amrith is obtained, we will steal the Amrith.

Thus, the great churning of the milk ocean began, with Mount Meru as the churner and the King of Serpents,  the Lord Vishnu's bed, the Great Naga, Vaasuki, was requested to be used as a rope.

From, advising the Good Devas to churn the ocean, the Divine Lord Vishnu, sided the Devas and protected them on time from every danger, proving that God is always with the Good, and he does not destroy evil,  but, controls the evil, so that, both Good and the Evil, co-exist, and one can learn to distinguish the Good from the Evil.

Lord Vishnu, had told the Devas to remain by the tail side of Vaasuki, so as to avoid the poison emerging   out of Vaasuki's mouth, and, the Asuras, stood by the head side of the Vaasuki, not knowing the poisonous  power of the Great Vaasuki.

As the great churning was taking place, the huge Mount Meru, began to slip through the Vaasuki ropes and  slide into the ocean, Lord Vishnu, took the form of a Tortoise (Koorma Avathaara) and went down the  ocean bed and lifted up the Mount Meru and balanced it.

After churning the milk ocean (Kshir saagar) for several years, first came the Halahala (poison), this poison  can destroy all the creations, so, without even thinking about himself, Lord Shiva, saw this poison emerging  out of the ocean and immediately began drinking it, Goddess Parvathi saw this extreme compassion of her  divine lord for the Good beings of the world, and, blocked his neck, by pressing it, so, the poison did not  enter Lord Shiva's stomach, instead stayed in his throat, and, his neck began turning blue, thus, he came to  be known as NeelaKanta, the one with a blue throat.

The results of churning ocean, gave rise to various things that came out of the ocean and a few are listed  here, not in any order: Surabhi, the Kamadhenu or divine cow, apsaras (angels), divine horse  (UcchaiShravaas), divine elephant (Airaavatham), vaaruni (half-angel half-demoness), kaustubha (divine necklace), parijatha (divine blossom tree with flowers), kalpaka vruksha (another divine tree), Shankh  (Vishnu's conch), Tulsi plant (Basil) etc.

Finally, Goddess Lakshmi emerged out of the ocean, and, the divine Lord Vishnu got enticed by her beauty  and offered to accept her as his consort to his divine abode Vaikuntam, Goddess Lakshmi accepted Vishnu  as her Lord.

The churning took place for several years and one fine day, it was Ashwayuja Month, Krishna Paksha  Dwaadhashi (dark fortnight 12th day), when, Lord Vishnu himself incarnated as Lord Dhanvanthri (the divine  physician) emerging out of the milk ocean with a Golden Amrith pot in his front right hand, ayurveda texts in  his front left hand, shankh (conch) in his back left hand and discus (sudarshan chakra) in his back right hand,  wearing a bright yellow cloth, wrapped around his waist, the lord was seen as a bright light emerging out of  a dark ocean, blue in colour, his eyes looking merciful and compassionate.

The Asuras got greedy and went to snatch the Amrith (Nectar of Immortality) from Lord Dhanvanthri, fierce  fighting began between the Devas and the Asuras, Lord Garuda (transport vehicle of Lord Vishnu) appeared  in all his majestic form, the divine and most powerful bird, took the Amrith from the Lord and flew away to  keep it safely. As, Lord Garuda flew with the nectar, 4 drops fell on the earth and these turned into holy  places of worship.

Lord Dhanvanthri, disguised himself as Mohini, the enchanting women, and, began dancing in front of the  Asuras and the Devas who were fighting, the Asuras got lured by her beauty and left the fight and began  dancing along with Mohini and following her, the Devas were stunned by her beauty and soon realized that  Mohini is their Lord Vishnu himself, and hence, sat in a row and sooner the Asuras too began sitting in a row  opposite the Devas, still looking at Mohini, Mohini was now seen holding the Amrith pot and she began distributing the nectar to the Devas, the Asuras were waiting patiently for their turn, when one of the asura  named SvarBhanu realized that something is going wrong and that the nectar may get over, and, silently,

 slipped sides and sat in between Surya(Sun) and Chandra(Moon) disguised as a Deva. The Devas did not  realize it, nor the Mohini who went about distributing amrith, SvarBhanu received his dose of the Amrith and  drank it quickly, Surya and Chandra suddenly looked on their side and were able to make out that it is an  asura in disguise and informed Mohini.

SvarBhanu was the son of Viprachit(father) and Simhika(mother), Simhika was the daughter of Hiranya- Kashyapa the great demon who was the father of Prahlaadha, the greatest devotee of Lord Vishnu.

Mohini, quickly changed her form into Lord Vishnu and threw Sudarshana chakra that cut off SvarBhanu's  neck, but, since he had already drank the nectar, he did not die, Lord Brahma made SvarBhanu's head and  body join with a snake, thus, the head joined a snake's body, came to be known as RAHU and the snake's head joined with SvarBhanu's body, came to be known as KETU.

Lord Brahma, granted RAHU and KETU a boon for accepting this new form, a place amidst the planets, the  Navagrahas. RAHU and KETU, even to this day, hold the grudge against surya and chandra, and cause  solar and lunar eclipse to affect them.

RAHU and KETU's power increases between sunset to sunrise, their power of vengeance is so strong that  they cause an astrological moment called the RAHU KAALAM, that stays every day during the day for 1 hr  and 30 minutes. This time is considered to be highly inauspicious for undertaking any good deeds and is only  good for propitiating the divine mother (Devi)