According to them the purpose of the incarnation was to contain the growing power of the Asura king, Mahabali and restore the lost glory of Indra, who suffered an ignominious defeat in his hands and lost his suzerainty over the heavens. Mahabali was the great grandson of Hiranyakasipu and grandson of Prahlada.

The Vamana (dwarf) Avatar was Lord Vishnu's fifth incarnation during the Treta Yuga. It was Vishnu's first incarnation as a human being. He reclaimed the gods' kingdom from Asura king Bali and returned it to Indra.

The Legend:

The majority of the story is based on the Vamana Purana. King Bali, also known as Mahabali, was Prahlada's grandson and Virochana's son. King Bali (also known as Mahabali) did penance to please Lord Brahma and received the boon of being undefeated in any battle. Following that, he attacked the heavens, defeating gods and driving Indra and other gods from heaven. He soon became the ruler of three worlds. Unlike the other Asuras, King Bali was a noble and kind-hearted man. He was fair to his subjects and took good care of them. As a result, all of his subjects were overjoyed with him.

The Birth of Vamana Avatar:

On the other hand, after being defeated by Asuras, the gods wandered around and were gloomed by King Bali's popularity. Indra, in particular, was disappointed because he was unable to defeat King Bali. Aditi, the mother of the gods, recognised the source of his sadness and performed a 10,000-year penance to appease Lord Vishnu. Aditi requested that Lord Vishnu retrieve the gods' kingdom from Bali and return it to Indra. Lord Vishnu agreed to her request. Aditi became pregnant, but soon after, the earth began to shake violently due to the unbearable weight of the foetus. Because of the enormous weight, that particular portion of the earth tilted down wherever she went. Her pregnancy had significantly diminished the demons' radiance.

This worried Bali, so he inquired of his grandfather, Prahlada, who meditated on Lord Vishnu and discovered that he would be born as Aditi's son. When he told Bali about it, the haughty king insulted Lord Vishnu. As a result, Prahlada cursed him, saying, "How dare you say such a thing! I am ashamed that you have taken birth as my grandson. Because you criticised Lord Vishnu, the supreme saviour, I curse you to lose your kingdom and fall from grace." Lord Vishnu was born as Aditi's son after some time.

Bali’s Yajna:

Bali once held a yajna and declared that he would give anything to anyone on that particular day. The Brahmin Vamana arrived soon after. He was wearing a kamandalu and a wooden umbrella. Bali inquired as to what he desired. Vamana instructed him to only give him three steps of land measured by his own feet. Bali cheerfully agreed. Vamana tilted his kamandalu to pour water on Bali's hands before accepting the donation. Recognizing that Vamana was not just any ordinary person, but Lord Vishnu himself, Shukracharya, the Asuras' teacher, took the miniature form and blocked the nozzle of the kamandalu. Vamana realised Shukracharya's trick and pushed a wooden stick inside the nozzle, claiming that an insect was blocking the flow of water. Shukracharya became blind in one eye after this stick pierced it.

Lord Vishnu Sends Bali to the Netherworld:

After that, Bali drank the water from Kamandalu. Vamana quickly grew to the size of a cosmos. He covered the entire earth in his first step. He covered the entire universe in his second step. There was no more space for his third step. As a result, he asked Bali where he should keep his third step. Bali realised that Vamana was none other than Vishnu himself. He told him that the only place left for Vamana to put his third step was inside his head. So Vamana took his third step on King Bali's head and pushed him into Patala (netherworld). Trivikrama Avatar is the name given to Vamana's giant form. Vishnu, pleased with Bali's generosity, made him king of the netherworld and bestowed upon him immortality. He permitted him to visit Earth once a year and meet his people. The Onam festival is held once a year when Bali returns to earth.

Asuras became weaker without Bali, and Indra reclaimed his heavenly kingdom.

Why did Lord Vishnu deceive Bali if he was so noble?

This could be due to one of three factors.

1. He had no choice because Lord Vishnu had given Aditi the boon that he would return heaven to the gods. He didn't assassinate Bali because he was a good ruler. As a result, he simply pushed him into the netherworld.
2. There is another version of Vamana Avatar's story. Although King Bali was noble for his people, he despised Vishnu. He was cunning, and by bestowing prosperity on people, he was destroying their devotion to Vishnu. Long ago, Lord Vishnu accidentally killed Shukracharya's (Asuras' teacher) mother while she was sheltering demons. As a result, Shukracharya became his adversary. He advised Bali to remove Lord Vishnu's devotion from the hearts of the people through deception rather than force. So Bali was doing something similar to Hiranyakashyapu, but in a different way. As a result, instead of defeating him in battle, Lord Vishnu deceived him.
3. Because of Lord Brahma's boon, Mahabali was unbeatable in battle. As a result, he had no other choice.