14-Shankhaasur

Shankhaasur Raakshas' body was of a conch shell.

This story comes from Padm Puraan - "Satyabhaamaa asked Krishn - "Why do you love Kaartik Maas and Ekaadashee Tithi, tell me the reason of this?" Krishn said - "Listen, In earlier times, King Prithu asked Naarad Jee a similar question. Naarad Jee said - "In earlier times there was an Asur named Shankh. He was the son of Samudra. He defeated all Devtaa, so Devtaa hid in caves. He thought that these Devtaa are powerful because of Ved, so let me take away their Ved, this will make them weak." At that time Chaaturmaasya (Shraavan, Bhaadrapad, Aashwin and Kaartik) was going on and Vishnu was sleeping in His Ksheer Saagar. Before He went to sleep all Adhi-Daivik forms of Ved also merged into the Divine water of Ksheer Saagar, that is why Devtaa were also very weak. Thinking thus he brought Ved from there and hid in the sea.

Brahmaa Jee went to Vishnu along with other Devtaa and prayed Him. Pleased with their worship, Vishnu said - "On Kaartik Shukla Prabodhinee Ekaadashee day, when only one Prahar (3 hours) night remains, whoever will worship me, they will come near me. Shankhaasur has kept all Ved in water. I will kill him and bring the Ved from him. From today, every year Ved and I will rest in water in Kaartik Maas. Since you have come on Ekaadashee, that is why this Tithi will be very dear to me. If anyone takes up Ekaadashee and Kaartik Maas Vrat, he will surely come to me."
[Padm Puraan, 5/12]