In Hindu mythology asuras were opposed to devas.
Both groups are children of Kashyapa. The name is cognate to Ahura – indeed, the Oxford English Dictionary recognises the use of the term in reference to Zoroastrianism, where "Ahura" would perhaps be more appropriate – and Æsir, which implies a common Proto-Indo-European origin for the Asura and the Æsir.
The negative character of the asura in Hinduism seems to have evolved over time. In general, the earliest texts have the asuras presiding over moral and social phenomena (e.g. Varuna, the guardian of rta, or Bhaga, the patron of marriages) and the devas presiding over natural phenomena (e.g. Ushas, whose name means "dawn", or Indra, a weather god).
In later writings, such as the Puranas and Itihasas, we find that the "devas" are the godly persons and the "asuras" the demonic. According to the Bhagavad Gita (16.6), all beings in the world partake either of the divine qualities (daivi sampad) or the demonic qualities (asuri sampad). The sixteenth chapter of the Bhagavad Gita describes the divine qualities briefly and the demonic qualities at length. In summary the Gita (16.4) says that the asuric qualities are pride, arrogance, conceit, anger, harshness and ignorance.
The Padma Purana says that the devotees of Vishnu are endowed with the divine qualities (viṣṇu-bhaktaḥ smṛto daiva) whereas the asuras are just the opposite (āsuras tad-viparyayaḥ).
The term asura corresponds to the Zoroastrian word Ahura. In Zoroastrianism the Ahuras are supreme, while the devas are demonic. This observation corroborates with some interpretations of the Indo-Aryan migration hypothesis, that a single tribe in Central Asia split into two parts, both ideologically and geographically, one migrating to India and the other to Persia.
In both cultures, this antagonism is worked out along the axis of sacrifice. The first Zoroastrian Gatha excoriates the worshippers of the daewas (devas) for their cruel treatment of sacrificial cows, while the Ahuras make efforts to protect the sacred cattle. In later Vedic ritual, the asuras and devas are frequently portrayed as fighting with one another over the offerings.
According to one hypothesis, the opposition between asuras and devas is rooted in proto-Indo-Iranian social structure. At important festivals, perhaps for new-year celebrations, it is postulated that two clans or sub-tribes would compete in making the most perfect ritual offering to the gods, seeking to outdo their peers in beauty of hymns sung, richness of offerings, and minute observance of traditional formulae. One clan would sacrifice to the devas, the other to the asuras. When proto-Indo-Iranian society grew and split, the two daughter societies slowly forgot the old agonistic context, and eventually chose one set of deities over the other.
Asuras also appear as a type of supernatural being in traditional Buddhist cosmology.