According to religious texts, Lord Vishnu took the form of Kurma (tortoise) and helped in the Samudra Manthan (churning of the ocean). Lord Vishnu's Kurma avatar is also known as Kachhapa avatar. Mount Mandarachal started rotating rapidly on the huge back of Lord Kurma and thus the Samudra Manthan was completed.
Lord Vishnu killed the demon king Hiranyakashyapa in the form of Narasimha. When Hiranyakashyapa was about to kill Prahlad himself, Lord Vishnu appeared from the pillar in the form of Narasimha and killed Hiranyakashyapa with his nails.
In Dwaparyug, Lord Vishnu destroyed the unrighteous by taking the incarnation of Shri Krishna. Lord Krishna also killed Kansa. In the Mahabharata war, he became Arjun's charioteer and gave the knowledge of Gita to the world. He established religion by making Dharmaraj Yudhishthir the king. This incarnation of Lord Vishnu is considered the best among all the incarnations.
According to religious texts, Lord Vishnu will take the form of Kalki in Kaliyug. Kalki Avatar will take place at the junction of Kaliyug and Satyayug. This avatar will be endowed with 64 arts. According to the Puranas, Lord Kalki will be born as the son of an ascetic Brahmin named Vishnuyash at a place called Shambhal in Moradabad district of Uttar Pradesh. Kalki will ride on a horse named Devadatta and destroy the sinners from the world and re-establish Dharma.