Namuchi was a very powerful demon who removed light from the universe and ushered in the era of darkness. It was Indra, the king of demigods, who killed Namuchi and saved the universe but the story has an interesting twist the head of demon Namuchi started chasing Indra around the universe. This story is found in the Mahabharata.
Namuchi had the boon that he would not be killed by anything that is wet or dry.
Due to the activities of Namuchi, the earth became barren and living beings began to perish. As there was not light, there was no food for the living beings.
Indra then approached Srihari Vishnu for a solution. Vishnu then collected foam from the milky ocean and made from it Vajra – a weapon that send out thunderbolts.
Indra used the weapon to annihilate Namuchi.
The time was evening. The severed head of Namuchi pursued Indra and cursed him, "O slayer of a friend, O wretch! May this sin damn you for all eternity.
Brahma advised Indra to take bath at the confluence of Aruna and Saraswati rivers and become pure. The head of Namuchi fell into the river and it perished.