There is a puranic legend relating to the origin of Rahu and Ketu. It is said that Lord Brahma had conferred the status of a planet to a celestial demon namely Swarbhanu. At some later stage, the Gods and demons collectively churned the Celestial ocean. There was a pact that everything resulting from churning shall be distributed equally among the Gods and demons.

At first impurities of the ocean (Poison) manifested. On the request of Gods, Lord Shiva drank the poison. After this, plants, animals, and other useful things came out. Ultimately Dhanvantri, the Celestial Physician came out with a Pot containing Amrita i.e. nectar known to confer immortality. The demons, because of their demonic nature, forgot about the pact. They immediately grabbed the pot of nectar from Dhanvantari and wanted to distribute it only among themselves, denying the Gods their share of the nectar.

Lord Vishnu was approached by the Gods for help. Lord Vishnu also realized that if the entire nectar is taken by the demons, it would mean that evil forces would gain permanence in the world. And there would remain no choice for goodness to thrive in the world. Lord Vishnu in fact desired to give nectar to the Gods only. To achieve this end, Lord Vishnu took the form of a beautiful lady. She enchanted the demons and made them agree to hand her the pot of nectar. She assured them that she would distribute it to them and the Gods as per their earlier pact. She asked the demons and the gods to sit in separate lines. The demons and the Gods agreed to it.

Lord Vishnu in the form of an enchantress started distributing the nectar first to the Gods. The plan was to distribute the whole nectar among the Gods, so that goodness may prevail permanently in the world. But demon Swarbhanu saw through the plan and stealthily sat among the ranks of the Gods and he managed to drink some of the nectar. Lord Vishnu knowingly overlooked his presence, as he was aware of the boon given by Brahma to Swarbhanu. Two of the Gods, Sun and Moon, discovered the presence of Swarbhanu in the rank of the Gods.

They got upset and requested Lord Vishnu to destroy him. Lord Vishnu severed Swarbhanus’ head with his Sudershan Chakra. But she could not kill him, as Swarbhanu had already partaken the nectar. The severed head was joined with a serpent’s tail and conferred by Brahma the status of a planet, came to be known as Rahu. While the remaining body was attached to the head of a serpent and given the planetary status with the name of Ketu.
Due to this incident, Swarbhanu, now divided into two (Rahu and Ketu) became a bitter enemy of the Sun and the Moon.