As told in the narration given to King Yudhishthira in the Mahabharata, Vritra was a demon created by artisan god Tvashta to avenge the killing of his son by Indra, known as Triśiras or Viśvarūpa.
Vritra won the battle and swallowed Indra, but the other gods forced him to vomit Indra out.
The battle continued and Indra was eventually forced to flee.
Vishnu and the rishis (sages) brokered a truce, with Indra swearing that he would not attack Vritra with anything made of metal, wood or stone, nor anything that was dry or wet, or during the day or the night.
Indra used the foam (which Vishnu had entered to ensure victory) from the waves of the ocean to kill him at twilight.

Srimad Bhagavatam recognizes Vritra as a bhakta (devotee) of Vishnu who was slain only due to his failure to live piously and without aggression.
This story runs thus:
SB 6.9.11: After Visvarupa was killed, his father, Tvashta, performed ritualistic ceremonies to kill Indra.
He offered oblations in the sacrificial fire, saying, "O enemy of Indra, flourish to kill your enemy without delay."

SB 6.9.12: Thereafter, from the southern side of the sacrificial fire known as Anvaharya came a fearful personality who looked like the destroyer of the entire creation at the end of the millennium.

SB 6.9.13-17: Like arrows released in the four directions, the demon's body grew, day after day.
Tall and blackish, he appeared like a burnt hill and was as lustrous as a bright array of clouds in the evening.
The hair on the demon's body and his beard and moustache were the color of melted copper, and his eyes were piercing like the midday sun.
He appeared unconquerable, as if holding the three worlds on the points of his blazing trident.
Dancing and shouting with a loud voice, he made the entire surface of the earth tremble as if from an earthquake.
As he yawned again and again, he seemed to be trying to swallow the whole sky with his mouth, which was as deep as a cave.
He seemed to be licking up all the stars in the sky with his tongue and eating the entire universe with his long, sharp teeth.
Seeing this gigantic demon, everyone, in great fear, ran here and there in all directions.

SB 6.9.18: That very fearful demon, who was actually the son of Tvashta, covered all the planetary systems by dint of austerity.
Therefore, he was named Vritra, or one who covers everything.

Vritra became the head of the Asuras (portrayed as inherently demonic here, as opposed to the Vedic version in which they can be gods or demons).
He renounced his dharma – duty – to do good unto others and turned to violence, battling with the Devas.
Eventually, he gained the upper hand and the Devas were frightened of his evil might.
Led by Indra, they approached Lord Vishnu for help.
He told them that Vritra could not be destroyed by ordinary means, revealing that only a weapon made from the bones of a sage could slay him.
When the deities revealed their doubts about the likelihood of any ascetic donating his body, Vishnu directed them to approach the rishi Dadhichi.
When approached by the gods, Dadhichi gladly gave up his bones for the cause of the good, stating that it would be better for his bones to help them attain victory than to rot in the ground.
The Devas collected the bones and Indra crafted the Vajrayudha from them.
When they engaged Vritra again, the battle lasted for 360 days before Vritra breathed his last.

As per the Puranic (Mahabharat) references, the terrible anthropomorphic personification of Brāhmanahatya (Brahmanicide) chased Indra and forced him into hiding for his sin, and Nahusha was invited to take his place.
As told in the narration given to King Yudhishthira in the Mahabharata, Vritra was a demon created by artisan god Tvashta to avenge the killing of his son by Indra, known as Triśiras or Viśvarūpa. Vritra won the battle and swallowed Indra, but the other gods forced him to vomit Indra out. The battle continued and Indra was eventually forced to flee. Vishnu and the rishis (sages) brokered a truce, with Indra swearing that he would not attack Vritra with anything made of metal, wood or stone, nor anything that was dry or wet, or during the day or the night. Indra used the foam (which Vishnu had entered to ensure victory) from the waves of the ocean to kill him at twilight.
Srimad Bhagavatam recognizes Vritra as a bhakta (devotee) of Vishnu who was slain only due to his failure to live piously and without aggression. This story runs thus:
SB 6.9.11: After Visvarupa was killed, his father, Tvashta, performed ritualistic ceremonies to kill Indra. He offered oblations in the sacrificial fire, saying, "O enemy of Indra, flourish to kill your enemy without delay."
SB 6.9.12: Thereafter, from the southern side of the sacrificial fire known as Anvaharya came a fearful personality who looked like the destroyer of the entire creation at the end of the millennium.
SB 6.9.13-17: Like arrows released in the four directions, the demon's body grew, day after day. Tall and blackish, he appeared like a burnt hill and was as lustrous as a bright array of clouds in the evening. The hair on the demon's body and his beard and moustache were the color of melted copper, and his eyes were piercing like the midday sun. He appeared unconquerable, as if holding the three worlds on the points of his blazing trident. Dancing and shouting with a loud voice, he made the entire surface of the earth tremble as if from an earthquake. As he yawned again and again, he seemed to be trying to swallow the whole sky with his mouth, which was as deep as a cave. He seemed to be licking up all the stars in the sky with his tongue and eating the entire universe with his long, sharp teeth. Seeing this gigantic demon, everyone, in great fear, ran here and there in all directions.
SB 6.9.18: That very fearful demon, who was actually the son of Tvashta, covered all the planetary systems by dint of austerity. Therefore, he was named Vritra, or one who covers everything.
Vritra became the head of the Asuras (portrayed as inherently demonic here, as opposed to the Vedic version in which they can be gods or demons). He renounced his dharma – duty – to do good unto others and turned to violence, battling with the Devas. Eventually, he gained the upper hand and the Devas were frightened of his evil might. Led by Indra, they approached Lord Vishnu for help. He told them that Vritra could not be destroyed by ordinary means, revealing that only a weapon made from the bones of a sage could slay him. When the deities revealed their doubts about the likelihood of any ascetic donating his body, Vishnu directed them to approach the rishi Dadhichi. When approached by the gods, Dadhichi gladly gave up his bones for the cause of the good, stating that it would be better for his bones to help them attain victory than to rot in the ground. The Devas collected the bones and Indra crafted the Vajrayudha from them. When they engaged Vritra again, the battle lasted for 360 days before Vritra breathed his last.
As per the Puranic (Mahabharat) references, the terrible anthropomorphic personification of Brāhmanahatya (Brahmanicide) chased Indra and forced him into hiding for his sin, and Nahusha was invited to take his place.
Vritra