One of Indra's famous battles was with the demon Vritra.
Vritra took the form of a fearful monster and stole all the water in the world. As the water disappeared, life on earth itself became threatened, but only Indra could save the world from this disaster.
Upon hearing what had happened, Indra pledged his help and went charging like thunder towards Vritra's fortress. As the building came crashing down, Indra saw Vritra standing in a corner. They clashed, and after a long drawn out battle, Indra killed him. He tore open the demon's body from which once again, all the water flowed out.
As the water flowed out, Indra was hailed as the king of the world.