Indra in hindu mythology is the Vedic god of rain and thunder.
He is king of the gods in the Rig Veda, ranking next to Agni. The Rig Veda also describes him as the great warrior, and leader of the Aryans, who crumbles the humble earthworks of the black, snub-nosed, “primitive,” of the land, loots the treasure houses of the “godless,” and “frees the rivers (a phrase taken to mean the breaking down of dams and levees).
It seems that Indra was originally a clan chief who was deified and over the centuries took on attributes of gods. At the height of the Vedic period he was a violent, hard-drinking Bronze Age barbarian; later on though he dropped to the second rank of the gods, his place ironically being taken by Krishna, a gentler god.
In early Vedic history Indra was thought of as a sky god. He gained his position by slaying Vrtra, or Ahi, the serpent of drought, who swallowed the cosmic waters and lay in coils around the mountains. Indra’s decisive thunderbolt split the stomach of Ahi, releasing the waters, generating life, and liberating the dawn.
The defeat of Indra, in Hindu mythology, at the hands of the demon Ravana, the Rakshasa king of Sri Lanka, and his release from captivity at the behest of Brahma was attributed to the seduction of Ahalya. But the tale of this humiliating punishment, as recounted in the Ramayana, may have been no more than the recognition of the decline of India’s celestial status, lowered perhaps by the Brahmins as a way of decreasing the influence of the divine patron of the warrior caste.
In the Chandogya Upanishad (c. 600 BC) the once powerful Indra, appearing as a fumbling student of the doctrine of Brahman, can apprehend the sophisticated teachings only with difficulty. Later in Hinduism, Indra becomes an aspect of Shiva, being subordinate to him, Brahma, and Vishnu.
Following Alexander the Great invasion of India (326 BC), the Greeks settling the area equated Indra with their god Dionysus. A.G.H.
Rice, Edward, Eastern Definitions: A Short Encyclopedia of Religions of the Orient, Garden City, New York, Doubleday, 1978, p. 186
Cotterell, Arthur, A Dictionary of World Mythology, New York, G. P. Putman’s Sons, 1980, pp. 72-73