During the churning of divine ocean, Amrita came out. While Devas and asuras fought for the amritha, Vishnu decided to give it to Devas. At that time an asura took the form of a Deva and drunk it. Knowing this, Chandra and Surya informed Vishnu about this. Vishnu hit the asura and killed him. The head of asura fell in Sirapuram which is now known as Sirkazhi and the body fell in other place which was known as Sempambinkudi and later became Sempankudi and Sempankudi. Due to consuming of Amrita, his body became two parts which in turn into two snakes. They worshipped Shiva to get rid from it. Shiva bestowed blessings and thus came into existence Rahu, with human head and snake body and Kethu with snake head and human body. Later they became one among the Navagrahas.