As per the Mahabharata, the nagas are the sons of Kadru, Daksha’s daughter, and Rishi Kashyapa, Brahma’s grandson. They are believed to live below the ground. They are of the human form up to the waist and of a snake below that, but they can change to a fully human or a fully serpent form whenever they wish. They have had an uneasy relationship with humans where they are sometimes friends and sometimes enemies. You could call them frenemies.
The most famous naga is Sheshanaga, who is the eldest of the thousand snakes born of Kadru. He has a thousand hoods, which he uses to support all the planets of the universe. Vishnu is often shown resting on Sheshanaga. When Sheshanaga uncoils, time moves forward and creation takes place, and when he coils back, destruction takes place. The younger brother of Shesha is Vasuki, who is commonly shown coiled around Shiva’s neck. Vasuki played an important role in Samudra Manthan. He coiled around Mount Mandara. The devas and asuras held one end each and churned the Ocean of Milk. Interestingly, Vasuki is known in Chinese and Japanese mythology as one of the eight Great Dragon Kings.
Manasa is Shesha and Vasuki’s sister. She is worshipped as a goddess in north and northeast India. She is believed to prevent and cure snake bites and grant fertility and prosperity. Manasa’s son Astika played a crucial role in saving the race of nagas from extinction. What had happened was that Parikshit, a Kuru emperor and son of Abhimanyu and grandson of Arjuna, had been bitten to death by snake chieftain Takshaka.
He did this to avenge an old act of Arjuna, in which the latter burnt the Khandava forest to clear land for establishing the new Pandava kingdom of Indraprastha. In this fire, Takshaka’s wife and millions of other nagas died.When Parikshit’s son Janamejaya became the emperor, he organised a huge snake sacrifice to finish off all nagas.
As the sacrifice began, snakes from all over were drawn into the fire. Astika, who was a learned saint, spoke to Janamejaya and the priests conducting the sacrifice, and was able to convince them to stop it, thus saving Takshaka and a few remaining nagas.
This happened on the Shukla Paksha Panchami day of the month of Sravana. Since then, this day has been celebrated as Naga Panchami. On this day, snakes are worshipped and offered milk by Hindus, Jains and Buddhists.
One idea related to nagas that has become wildly popular is that of ichchadhari naag (shape-shifting cobra). Comic book superhero Nagraj, TV series like Naaginn and Bollywood films like Nagin and Nagina are based on the concept.