Vritra was a demon (Asura). The story about Indra, the king of god, killing him is very popular in the ancient Indian epics. It is so old that we can find mention of it even in the Vedas. The story is basically too simple to arouse any interest. Its importance is primarily because of its antiquity and the fact it is told again and again.
There are different versions of the story. In one version Vritra was the son of sage Kasyapa and in the other, of sage Twashta (twashTa). In all versions his mother was a demon.
There is reference to this story at least on three different occasions in Mahabharata. The first was in the chapter of ‘Pilgrimage (Teerthayatra Parvam)’ of the ‘Book of Forest Life (Vanaparvam)’. The second is in the chapter of ‘Army Deployment (Senodyoga Parvam)’ of the ‘Book of war preparations (Udyogaparvam)’. And finally, in the chapter of ‘Steps towards Salvation (Moksha dharma anuSasana parvam) of the Book of Peace (Saanti parvam)’
Vritra joins the demons
Vritra, though of mixed birth, had his affinity with demons, his mother being one of them. So, it is said that he went about fighting against the gods and destroying the sacrificial rituals in which offerings were made to them.
In the popular version, the demon meditated on Brahma and won a powerful boon. By that he could not be killed, during the day or at night, by any weapon made of metal.
A weapon made of bones
The gods under the leadership of Indra fought several battles against Vritra. The demon was so powerful and the boon he got so potent that he remained invincible. It was Brahma who suggested to Indra that a special weapon, Vajra (literally meaning a diamond or the thunderbolt), be made with human bones. He further said that a sage by name Dadheeci (dadheeci) was spending his days in deep meditation with the singular aim of attaining salvation. He would gladly give his bones for the purpose. (Not clear why the bones of a living person were required).
When Indra approached the sage he was only too happy to give up his life for a noble cause. That was how the powerful Vajra was made of the bones of sage Dadheeci.
Vishnu gave the idea to Indra that the king of gods should attack the demon at dusk, to avoid both the day and night times. Indra did accordingly. The weapon made of bones, which was not metallic, and the time of the setting sun found the loop holes in the boon the demon had obtained. Indra dispatched the powerful Vajra against Vritra and brought his head down for the relief of all gods.
The sin that inflicted Indra
It is said that by killing Vritra who was the son of a Brahmin Indra was vulnerable to the sin called Brahmahatya. Indra got so frightened that he hid himself inside the stalk of the lotus flower on which Brahma usually sat. Nothing would make him to come out of his place of hiding. The whole world suffered as a result, he being the king of gods. So, Brahma pleaded with four elements found in the nature, like fire and water, to equally share the sin and thereby rid Indra of the malady. The elements of nature agreed to accept the sin on condition that those who pollute nature get a share of it. For example, one who pollutes water gets a small share. Brahma, thus, could rid Indra of the sin of killing Vritra.
The three interpretations
Many of the stories found in the epics and Vedas are interpreted at three different levels: the physical or the worldly level (aadhibhautikam), the heavenly or the divine level (aadhi daivikam) and the spiritual or the intellectual level (aadhyaatmikam). This is possible, because the root of the words Indra and Vritra permits different interpretations.
At the physical level Vritra is the cloud and Indra the rainmaker. The Vajra, the weapon of Indra, was the lightning or the thunder. The rainbow is called the bow of Indra (Indradhanus). The killing of Vritra is simply the rain bursting out of the cloud.
At the divine level the god, Indra, kills the demon called Vritra as the story is normally told.
At the spiritual level, Vritra means darkness, caused by ignorance. Indra is enlightenment. Enlightenment removes the cover of ignorance that engulfs human mind. There is one school of thought that upholds this interpretation of the story in the context of the Vedas.