In Hindu mythology, the Asura are a group of power-seeking deities, sometimes misleadingly referred to as demons. They were opposed to the devas. Both groups are children of Kasyapa. The name is cognate to Aesir and according to linguists this shows a common Proto-Indo-European origin for the Asura and the Aesir.
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).
The term corresponds to the Zoroastrian word Ahura. In Zoroastrianism the Ahuras are supreme, while the devas are demonic. This observation corroborates with the "Aryan invasion" hypothesis that a single tribe in Central Asia split into two went separate ways, 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.