Narasimha Avatara is the fourth Avatara of Lord Vishnu. The story of Narasimha Avatara is also mentioned in Padma Purana’s Uttara Khanda.
Once, goddess Parvati expressed her desire to know about all the incarnations of Lord Vishnu.
Continuing with the various incarnations of Lord Vishnu, Lord Mahadeva told Parvati —
Sage Kashyapa had two mighty sons from Diti — Hiranyakashipu and Hiranyaksha. Hiranyaksha had once carried the earth to Rasatala. The deities requested Lord Vishnu to rescue the earth. Lord Vishnu took the incarnation of a boar and killed Hiranyaksha with his sharp horn.
When Hiranyakshipu learnt about his brother’s death he went to Merugiri mountain and started doing penance to please me (Lord Shiva). I blessed him with immortality, Later on, Hiranyakashipu married Uttanpad’s daughter–Kalyani and had a son named Prahlada from her. In spite of being born in the clan of a demon, Prahlada proved to be an extremely religious child and was a supreme devotee of Lord Vishnu.
When Hiranyakshipu came to know about Prahlada’s religious tendency, he tried his best to stop him from worshipping Vishnu. But, all his efforts went in vain and Prahlada continued to worship Lord Vishnu.Hiranyakashipu instructed his subordinates to kill Prahlada but each time he came out unharmed.
One day, Hiranyakashipu pulled out his sword and said—‘You say that Vishnu is omnipresent. Is he present in this pillar?’ Having said like this he assaulted the pillar with his sword. Suddenly to his sheer amazement, Lord Vishnu manifested himself from that pillar in the form of Nrisimha.
Hiranyakashipu had been blessed by Lord Shiva that he could be killed neither by a man nor a beast. No weapon could harm him. He could be killed neither during the day nor during the night. Hiranyakashiu had taken into consideration all the possible threat perceptions to his life and tried to become immortal. But, Lord Vishnu takes incarnation to protect the humanity whenever religiousness is under threat.
This time, Lord Vishnu took incarnation as Nrisimha (partly lion and partly human) and tore apart his belly with his sharp nails. After the death of Hiranyakashipu, Prahlada was coronated as his successor. Prahlada ruled justly and his subject were happy and satisfied.