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The priest said "You are the ruler of the three worlds: the Earth, the world above the skies and the underworld. Would you give me the space that I could cover with three strides?" King Bali laughed. This request seemed strange to all the courtiers, but king Bali granted it. Surely a dwarf could not cover much ground, thought the King, who agreed to dwarf's request.
Once the request was granted to him, the priest, who was none but Visnu in disguise, submerged the entire world (Mrityu-loka) with one step, with the...
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second he submerged heaven (Swarga-loka) and for the third step; there was no respectable place to put his foot down and so he asked the bewildered Bali for some respectable place to this foot so that the boon could be fulfilled. Left with no alternative, the noble but exasperated King Bali offered his head for the purpose. To his surprise, the priest not only lost no time in placing his foot over King Bali's head, but also thrust Bali into the nether worlds (Patala-loka) which as per Hindu cosmogony is hell and lies below the surface of the earth.
The second day of Diwali (Bali Prati-pada) is celebrated in memory of this. Prati-pada here translates as "below the opponent's foot" (Prati=opponent, Pada=foot). During Diwali festival people celebrates the victory of good over evil.
(source: www.diwalicelebrations.net/diwali-legends/bali.html)