Even though Bali was just, the kingdom of the gods had to be returned to them, who felt humiliated at the loss. Lord Vishnu could not fight with such a virtuous king, and therefore he took the guise of a Brahmin boy.
Bali was performing a yagna and for the success of the yagna he had to provide anything that a brahmin seeks. Lord Vishnu in his Vamanavataram begged for just three feet of land. This request was considered funny, for three steps of Vamana's feet would measure upto a very small piece of land only. Bali humbly requested him to ask for something better which he would grant. But Vamana insisted on his three feet of land. To the astonishment of all present, Lord Vishnu as Vamana began to grow to a great height and with one step he covered the whole earth, and with his second step he covered the entire sky and region above.
With the entire universe covered, Bali had no more land to offer. He bowed down in front of the Lord and offered his head for the third step. Lord Vishnu had thus regained the power from the asura king. Being extremely pleased with the virtuosity of an asura king, he placed his foot on his head and sent him to rule over the lower regions.
There are many temples all over India with the form of Vamana dev being worshiped especially in Kanchi.
BALI, the grandson of Prahlada was a very valorous and mighty asura. By his penance and might, he conquered the whole world. Indra and other gods fearing that he and the asuras would conquer all the three worlds, went to Lord Vishnu for help. Lord Vishnu was then born as a dwarf Vamana in the household of a brahman priest. He went to Bali on growing up and asked for alms. Bali was delighted to offer him anything he requested even though his royal guru warned him that it was Lord Vishnu. Vamana then requested for the amount of land that could come under his three feet. Bali gracefully agreed. Lord Vishnu then grew in size and covered the earth and heaven in two stride. And due to lack of space, he put his third leg on Bali himself and crushed Bali to the nether or the Patala loka (underground world), thus helping the Gods out.
