The Bhagavata Purana describes that Vishnu descended as the Vamana avatharam to restore the authority of Indra over the heavens, as it had been taken by Bali, a benevolent Asura King. Bali was the grandson of Prahlada, the son of Virochana. King Bali was generous, and engaged in severe austerities and penance and won the praise of the world. With the praise from his courtiers and others, he regarded himself as the all-powerful in the world.
Vamana, in the guise of a short Brahmin carrying a palm leaf umbrella, went to the king to request three paces of land. Bali consented, against the warning of his guru, Sukracharya. Vamanan then revealed his identity and enlarged it to gigantic proportions to stride over the three worlds. He stepped from heaven to earth with the first step, from earth to the netherworld with the second. King Bali, unable to fulfill his promise, offered his head for the third.
Vamana then placed his foot and gave the king immortality for his humility. Upon worshiping Mahabali and his ancestor Prahláda, he conceded sovereignty of Rasatala. Some texts also report that Vamana did not step into the Rasatala, and instead gave its rule to Bali and granting him the boon to become the next Indra. In giant form, Vamana is known as Trivikrama. The legend is associated with the temple and also with Ulagalantha Perumal Temple, Tirukoyilur and Ulagalantha Perumal Temple, Kanchipuram.
As per another legend, a rich man who owned a plantain garden, did not get much yield from it. He donated a set of plantain to the temple, after which it started yielding as golden plantains. The groove came to be known as Nentiram Palam. The ruling king became suspicious of the change that it was his wealth in the garden and tortured a sage. The sage cursed the king. The king was afraid and in redemption, he consulted a group of sages. They suggested a thatch of bamboo and created a light with "Koraipul", a grass which helped him redeemed from the curse.
Onam festival
Thrikkakara temple is considered to be a centre of Onam celebrations, as Thrikkakara is considered to have been the abode of the King Mahabali. The celebration of the Onam festival is the main event in the temple. The festival is celebrated over a period of ten days in the Malayalam month of Chingam. The temple houses the main deity Lord Vamana. During the Onam celebration period, a pyramidal statue idolising Maveli and Vamanan is installed as a symbol of honour at all other sites of the celebration, and named Thrikkakara-appan. The temple is the site at which the king Mahabali is said to have been sent to the netherworld Patala by Vamana with his foot, hence marking the genesis of the Onam festival. The etymology of the name Thrikkakara ('Thiru-kaal-kara' meaning 'place of the holy foot') is also derived this way. Some features of the Onam festival at Thrikkakara are
Flag-hoisting and lowering: The temple festival begins on the first day (Atham) with the Kodiyettu (കൊടിയേറ്റ്) ceremony, which is a flag-hoisting ceremony common in festivals in temples in Kerala. The festival ends on the 10th and final day, which starts off with a symbolic welcome of the Asura king Mahabali. The closing of the festival is marked with the lowering of the flag and bathing of the idol, referred to as Aarattu (ആറാട്ട്).
Atthachamayam procession and finale: The festival is flagged off all over Kerala by a grand procession beginning at Tripunithura near Kochi called Athachamayam (അത്തച്ചമയം). In olden days, the Kochi Maharaja would head a grand military procession in full ceremonial robes from his palace to the Thrikkakara temple.
Onam feast: A highlight of the festival is the grand banquet, or Sadya, held on the last two days of the festival at the temple campus. The feast has grown significantly in magnitude each year, and is currently attended by more than twenty thousand people. People belonging to different faiths and religions turn out in large numbers for the sadya in keeping with the spirit of the festival.
Decoration of idol: The Chaarthu (ചാർത്ത്) is a form of decoration of the Vamana idol using mainly sandalwood paste, ornaments and clothing. On each day, the idol is decorated in the form of one of the Ten Avatars of Vishnu, including the Matsya (fish), Kurma (Tortoise), Varaha (boar), Narasimha (half-man half-lion), Vamana, Parashurama, Rama, Balarama, Krishna, Kalki and Trivikrama (another form of Vamana).