Updated: Dec 9, 2022
The asura king Bali, once defeated Indra, the king of the gods. All the gods then paid a visit to Vishnu, to help them restore the order of things.
Meanwhile, Bali ruled the realms with benevolence, conducted yagnas & gave out alms to the needy. During one such event, Vishnu came to visit as Vamana (dwarf). He asked the king for land that his three steps could cover, which was readily agreed by Bali.
No sooner had Bali agreed that Vamana started to grow size and became even bigger than the earth! He placed his first step on the earth & second on the heavens (Indra's kingdom), thereby claiming both as his own. There was no place left for him to place his third step. Righteous that he was, Bali offered himself and asked Vamana to place his third & last step on his own head. Impressed by his nobility, Vishnu blessed him be the king of the underworld (Paatal Lok) also gave him a boon to be able to visit his land and people, once every year, which is today celebrated as Onam.