The race of snakes trace their origins to Kadru, the wife of rishi Kashyapa. Kadru bore a thousand snakes for her sons through the grace of the muni. Kadru also had a sister by the name Vinata, who was also married to Kashyapa. Vinata had a son by the name Aruna, who ended up becoming the Sun’s charioteer.
One day, the sisters beheld the celestial white horse Ucchaishrava flying in the sky. They entered into a wager about the colour of the horse’s tail. Vinata claimed that it would be white, while Kadru bet that the white horse had a black tail. They agreed that the loser of the wager would spend her life in servitude to the other sibling. Deciding to ascertain the colour of the tail the next day, the sisters returned home.
At home, Kadru requested her sons (snakes) to seek the horse overnight and attach themselves to the tail so that it would appear black during inspection. The snakes refused. Kadru cursed them and declared that they would be burnt in the fire of the yagna conducted by King Janamejaya. Terrified by this curse, the snakes decided to appease her and attached themselves to Ucchaishravas’ tail.
The next day the sisters went to inspect horse. Observing a black tail, Vinata accepted defeat and started serving Kadru.
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After a while, Vinata bore a winged son. He was none other than the King of birds, Garuda. The snakes would task Garuda with chores, and Vinata would ask her son to duly oblige. Garuda wondered why he had to bide by the orders of the snakes. He soon learnt that his mother was spending her life in servitude due to a lost wager.
Wanting to set her free, Garuda went to the snakes and put forth his request. The snakes agreed to free his mother, provided Garuda fetched them a pot of divine nectar (Amrita) in return.
* * *
Garuda then embarked on an adventurous quest. As per the advise of his mother, he first reached an island and fed on Nishadas (a tribe of fishermen) living in that place (Garuda did not have any food sanctioned at that time by the Creator). He then preyed upon a vengeful and monstrous pair of an elephant and a tortoise. In the process, he also saved a group of Valakhilya rishis falling upside down from the branches of a huge banyan tree. During this time, he met his father Kashyapa who was then meditating in the woods and received his blessings .
Later, he proceeded to claim the nectar of immortality from the celestials. A fierce battle ensued, where Garuda defeated Indra, Sadhyas, Gandharvas, Vasus, Rudras, Aswin twins and Yakshas. Garuda then doused a huge column of fire on his path, before entering the place where the Amrita was guarded.
The nectar was guarded by a spinning wheel-like device intended to cut any intruder to pieces. Garuda assumed a miniature form and entered the device. He then came across two fiery snakes, whom he cut into pieces and successfully claimed the pot of elixir.
Garuda, without consuming even a drop of nectar, rushed back to the abode of the Nagas. Observing his selfless act, Vishnu became appeased. He appeared before the bird and offered him a boon of his choice. Garuda asked for immortality and also an eternal position above the Lord (in his flag). Vishnu agreed. Garuda too offered Vishnu a boon in return, and the Lord hence called for Garuda to become his vehicle. Garuda acquiesced, and subsequently took to the skies.
Indra, observing Garuda flying with great speed, hurled his Vajra weapon towards the bird. Garuda, despite being struck with the weapon, smiled and told Indra in polite words, “O King of the Devas. I respect the rishi (Dadichi) from whose backbone this Vajra weapon was made; I respect you too. To honour this, I will shed a single feather. But know that I have not felt any pain due to this weapon”. Saying this Garuda dropped a single feather. Beholding this act and seeing the beautiful feather, all the creatures became excited, and said, “Let this bird of beautiful feather be called ‘Suparna’ (one of fair feathers)”.
Indra, realizing the bird’s greatness, sought Garuda’s friendship and asked him not to share the nectar with anyone. Garuda replied, “O King, I am taking this to my destination for a reason. Once I deliver this, you can swiftly come and take this away.”
Indra agreed, and offered Garuda a boon. Remembering the deceitful acts of the snakes that led to his mother’s slavery, Garuda said, “Let the snakes become my food”. Indra said “So be it”.
* * *
Upon reaching the destination, Garuda placed the nectar on kusa (darbha) grass. He told the snakes to take their purifying bath & auspicious rites before consuming the nectar. He also asked them to liberate his mother. The snakes agreed. When they stepped away for their bath, Indra swiftly took back the pot of nectar.
The snakes came back and saw the pot missing. Thinking that drops of nectar may have come into contact with the kusa grass, they started licking the grass. The sharpness of the blades of grass cut their tongues due to which the organ acquired a forked shape. In addition, due to the contact with Amrita, the kusa grass became sacred from that time.
(Source: Adi Parva, Chapters 16 to 34)