In the earliest Hindu stories that we know of, from the Rig Veda, Indra is the ruler of the Hindu gods, the way Zeus is the ruler of the Greek gods, or Thor the ruler of the German gods. These stories probably go back at least to about 1000 BC. In the Rig Veda, Indra is the son of Dyeus Pita, the Indo-European god from whom the Greeks got Zeus, the Romans got Jupiter, and the Germans got Tyr. Indra may be the same god as the Greek god Dionysos, also a son of Zeus, though Dionysos is not a storm god. Indo-European gods often overthrow their fathers, as Zeus overthrew Kronos, and apparently in India people thought that Indra killed Dyeus Pita and took his powers.
Indra’s main weapon was the thunderbolt, the same as Zeus. Indra rode a white elephant called Airavata. Sometimes carvings of Indra show him with four arms. Indra lived in the clouds near the top of a tall mountain. After warriors died, they could go up to Indra’s house, where they lived forever, playing games and watching dancers perform. People worshipped Indra by sacrificing animals to him.
But by around 300 BC, when Chandragupta united India into the Mauryan Empire and the Silk Road got started, people in India didn’t like to sacrifice animals any more, and they gradually stopped worshipping Indra so much. Instead, they began to worship the new gods Shiva and Vishnu, who didn’t need animal sacrifices. This was part of a turn against sacrifice all over Eurasia; Christianity, which replaces animal sacrifice with the sacrifice of Jesus, is one example, but the Zoroastrians, the Chinese, the Greeks, and the Romans also gradually stopped sacrificing so much. But people still thought of Indra as a god, and told stories about him and how he lost his power.