Vritra

He appears as a human-like serpent blocking the course of the Rigvedic rivers, and is slain by Indra with his newly forged vajra.

The combat began soon after Indra was born, and he consumed a large volume of Soma at Tvashtri's house to empower him before facing Vritra.

[8] As told in the narration given to King Yudhishthira in the Mahabharata, Vritra was an asura created by the artisan god Tvashtri to avenge the killing of his son by Indra, known as Triśiras or Viśvarūpa.

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.

The Srimad Bhagavatam recognizes Vritra as a bhakta (devotee) of Vishnu[9] who was slain only due to his failure to live piously and without aggression.

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.

The hair on the demon's body and his beard and moustache were the colour of melted copper, and his eyes were piercing like the midday sun.

[11] Vritra became the head of the asuras (portrayed as inherently malicious here, as opposed to the Vedic version, in which they may be benevolent or malevolent).

When the deities revealed their doubts about the likelihood of any ascetic donating his body, Vishnu directed them to approach the rishi Dadhichi.

Even as the devas gasp, Vritra merely advises him to pick up his weapon, since life and death are the same for him, as he believes that they are all instruments of Vishnu.

Protected by Vishnu, Indra cuts open the belly of Vritra and escapes, finally beheading him with the vajra.

[12] According to the Puranas, the terrible anthropomorphic personification of Brāhmanahatya (Brahmanicide) chased Indra and forced him into hiding for his sin,[13][14] and Nahusha was invited to take his place.