History of Onam
Mahabali was an Asura King who with his energy prevailed over the underworld and also the sky. After the thrashing from King Mahabali and losing the paradise, devas loved Lord Vishnu to help them restore their lordship over the paradise.
Tolerating the solicitation, Lord Vishnu took the mask of a little Brahmin kid by the name Vamana and approached Bali for land that he could cover with his 3 steps. Tolerating the solicitation, King Mahabali conceded Vamana the aid to take the area he could cover inside his 3 steps.
After getting the boon, Vamana developed to huge size and secured the earth and underworld with his first step. With the second step he secured the paradise leaving King Mahabali in a fix as no room was left for the third step. With this, King Mahabali offered his head to Vamana and requested that him put his third venture on his head. Setting the third venture on the King’s head and satisfied with his liberality, Lord Vishnu proclaimed King Mahabali the ruler of Patal Lok and gifted him the boon of visiting his people once every year.