The reign of King Mahabali is considered to be the golden era. There were no lies, cheating, poverty – it was utopia in reality. Due to this the demon King Mahabali ruled the entire universe and the Devas (Demi gods) had lost their power. Lord Vishnu had to intervene to save the Devas. He took the form of dwarf Vamana and attended a yagna being conducted by King Bali. Vishnu in this form is also known as Trivikrama.
During the yagna, Mahabali made an announcement that he will fulfill any wishes made by people. Vamana was waiting for this opportunity and asked him to grant as much land as he could step over in three steps. Mahabali granted the request thinking how much land can the dwarf take using three steps, when he has the entire universe in his disposal.
Suddenly Vamana took a gigantic form and in one step covered the whole heaven, and with the second step the earth and netherworld (Pathalam). As there was no more space left to keep the third feet, Mahabali lowered his head and Vamana placed his foot on it and sent Mahabali down below to rule the netherworld (Pathalam).
Lord Vishnu also gave a boon to Mahabali and King Bali asked that he be allowed to visit his people once in a year. The Onam festival celebrates the annual visit of Mahabali.