Devas wanted to drink Amrit, so that they will become immortal. That's why they made an agreement with Asura to churn the sacred ocean to get Amrit. But to make Devas only as immortal, Mahavishnu become an Apsaras and cheated Asuras and distributed Amrit only to Devas.

When an Asura Swarbhanu came to know about this cheating, he disguised like a Deva and drank Amrit. So that, to kill him Mahavishnu cut his head with his Sudarson Chakra. But as he already drank Amrit, he was not dead. So that, his head become Rahu by attaching a snake's body and his body become Ketu by attaching the snake's head. Later, these Rahu and Ketu become Chaya Grahas in Astrology.

In Astrology, all planets move in forward direction whereas Rahu and Ketu moves in backward direction. Surya, Chandra, Angaraka, Budha, Brahaspati, and Shukracharya are Devas. Rahu and Ketu are Asura. Shani is neither Deva nor Asura.

When Rahu and Ketu were given the task of protecting Vedas, Rahu protected Rig, Yajur, Sama Vedas and Ketu protected only Adharwan Veda. In that time, Rahu and Ketu studied Vedas in their respective custody. By the way, Rahu become more materialistic and Ketu become detached from all materialistic pleasures and signifies Sannyas.

As Rahu denotes materialistic pleasures, if he is well placed and Rahu mahadasha comes in prime age (25-60), then it will give the native a Raja Yoga and a luxurious life. Rahu also gives the native greed, anger, new friends, and interest in studying new languages.

If Rahu is afflicted, it will give the native confused mind, anxiety, cheating friends, and many obstacles in career, severe illness, death of close relatives and many more malefic results.

Many says that wherever Ketu sits, he will destroy that house. But it is not true. Wherever Ketu sits, the native will be detached from the house related things and that will be best for the native. If Ketu sits in 5th house, the native's child birth will be delayed or the native will be detached from the kids. It will be good for both the native and the kids.

If Ketu sits in 2nd house, the native will not be more interested earning money and accumulating wealth and this nature will be more perfect for the native. If the native become greedy, Ketu starts giving him more obstacles in earning wealth. Many astrologers says, if Ketu sits in 12th house, it is the last birth for the native and there will be no rebirth. But as I think, it is not true because Moksha is not that much easy.