The story of Onam festival is associated with the Vamana
incarnation of Vishnu and King Mahabali (Maveli). Onam, the most popular
festival in Kerala, is the day when the legendary Asura king Mahabali visits
his subjects. Mahabali had turned his kingdom into a virtual paradise on earth
and the ‘devas’ could not tolerate this and they sought the help of Lord Vishnu
to eliminate Maveli. Onam is perhaps a rare Hindu festival in which Lord
Vishnu, the vanquisher, is the villain and the vanquished King Mahabali the
hero.
As per Puranas, the Asura or demon King Mahabali made the
utopian kingdom a reality. His fame spread all over the earth and the
netherworld and slowly into the heaven.
Indra, the king of Devas and the ruler of heaven, felt
threatened by the growing popularity of Mahabali.
Aditi, the mother of Indra, observed a penance called
'payovrata' to help her son and pleased with her devotion Lord Vishnu was born
as Vamana, a dwarf, to Aditi.
Vamana approached Mahabali who was conducting a yajna. With
the completion of this Yajna, King Mahabali will get the authority to overthrow
Indra.
King Bali asked Vamana to choose anything that he wanted
from his kingdom. Vamana asked for just three steps of land. And the king
agreed readily.
Suddenly, the dwarf sized Vamana grew into the skies and
with one step covered the earth and with another step the heavens. With no
place to keep the third step, Mahabali offered his head, which was pushed by
Vamana into the netherworld or pathalam.
But before sending Mahabali to the netherworld, Lord Vishnu
gave Bali the boon that he can visit his subjects once in a year. The
day Mahabali visits his subjects is celebrated as Onam.
The story in detail can be found in the 8th canto of the
Srimad Bhagavatam.