Rahu and Ketu
The story of Rahu and Ketu is as follows:
Long ago there was a great war between the Asuras (demons) and Devatas (gods). The Devatas were routed in the war and they went to Lord Brahma to seek his help. Brahma told them to churn the ocean.
As a result they would be able to obtain Amrit (nectar) which would make them immortal and all powerful and after consuming it they would be able to defeat the Asuras. Churning the ocean was a very big job and so the Devatas sought the help of the Asuras. The king of the serpents Vasuki was to become the rope, while a hill named Mandrachal offered to become the pivot. Lord Vishnu, in the form of a tortoise, became the base and the churning process began.
The Devatas held the tail of Vasuki and the Asuras held the head. Various things came out, during this churning of the ocean. There was a pot of Vish (poison) which created havoc in all the three worlds. Lord Shiva drank the poison to save the world, but he did not let the poison go beyond his neck which became blue in color. That is why Lord Shiva is also called 'NeelKant' ("one with a blue neck").
At last Dhanvantari came out with a pot of Amrit and immediately there was a mad scramble for it among the Devatas and Asuras. Seeing this, Lord Vishnu took the form of Mohini (a beautiful dancer) and offered her services in distributing the Amrit equally to both the Asuras and Devatas. She made both of them sit in two different rows but she gave the Amrit to the Devatas only. Towards the end, one of the Asuras saw through her trick. He went and sat among the Devatas and drank the Amrit. The moment Chandra (Moon) and Surya (Sun), who were sitting besides him, saw that he was a Asura, they informed Mohini. Lord Vishnu came in his real form and let out the Sudarshan Chakra (wheel) at the Asura. The Asura's neck was separated from the body, but he did not die as he had drunk the Amrit. His head was called 'Rahu' and his torso 'Ketu'. According to legend, Rahu and Ketu swallowed the moon and sun to have their revenge, causing an eclipse.