Bali is a demon king, who is grandson of Prahalad and a great devotee of Lord Vishnu. Bali also known as Mahabali was about to cause mayhem on the earth. Bali was a greedy king but on the other hand he was a great philanthropist. He captured all the beautiful women for his pleasure and thus caused panic in the earth.
Lord Vishnu came to rescue the world from Bali`s torture. He appeared as his `avatar` or consort of Vamana or a dwarf Brahmin boy. The boy asked for land from King Bali. Bali asked that how much land he needs. The Boy answered that only three steps of land would be enough for him. Bali smiled and granted his prayer. The Vamana covered the whole earth with his first step, put his second step into the sky and wanted to know from Bali Raja where he should keep his third step. Bali realized who this boy was and offered his head so that Vamana could put his third step there. Thus Vishnu saved the world.
Bali renounced his kingdom to keep his word and went to the underground or `Patala`. Hindus believe that Bali is immortal.