Kaal means Time and Sarp means Serpent. KaalSarp, therefore, means the Serpent of Time. This Yog is formed if all the planets are between Rahu and Ketu. When that is in one's birth–chart, the person affected in his life by many ups and downs.

I want to tell you one interesting thing according to our vedic history Maharshi Kashyap had 13 wives, daughters of Daksh Prajapati.The famous Aditi and Diti, Kadru, Danu, etc. Diti have a daughter named Singhika, married with Viprachhit. Singhika had a mighty son whose name was Swarbhanu. He performed severe penance and Lord Brahma was pleased. When he wanted to grant him his wish, Swarbhanu wanted to have the same status in the heavens as the planets. The boon was granted.

After some time the Gods and the demons churn the ocean in search of nectar. The mountain " Mandhara " was the churning stick and " Vasuki “, the king of serpents, was the churning cord.
A problem arose. Mainak was slipping off to the ocean floor whenever churning was started. To prevent this, Lord Vishnu took the form of a tortoise and Mandhara was placed on its back. The churning started and many things started coming out, when at last Amrit (Nectar) came up. Amrit means ‘No death' makes one immortal. Gods and demons were mad for it and before anything could happen. In the guise of a beautiful woman (Mohini) took it in her custody and asked Gods and demons to sit in two rows, so that she could distribute Amrit to them. The gods knew what was going on and the demons that did not. The Gods sat in a line on one side and the demons on the other side. Mohini started with the Gods.
The demons charmed as they were, failed to see the trick. But Swarbhanu, son of Singhika, with his huge head and body, ending in a serpent like tail, was smart enough to smell a rat. He broke the row and went to sit with the gods. Amrit was served and Swarbhanu was about to take it when Sun and Moon found out that there was an imposter among them. They alerted to Lord Vishnu and he severed Swarbhanu's head with his “ Sudarshan Chakra ”. But alas! Nectar had already touched his lips and did its job. The  “Sudarshan Chakra” had separated the head and the tail and both had, by the drink of the nectar, become indestructible. Now Lord Brahma's boon was granted and the head and the tail were given the status of planets and placed in the heaven. Head called “ Rahu”(Dragon's Head) and tail called “Ketu”(Dragon's tail)

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Rahu Yantra