Narsimha is an incarnation of Lord Vishnu,according to Hindu scriptures Lord Vishnu took the incarnation to protect his devotee Pralada. Prahlada was the son of demon king Hiranyakashipu who hated even the name of Lord Vishnu,due to this haterdness he even didn't spare his son Prahlada.Prahlada had to face lot of torture and abuse by his father.   Hiranyakashipu tried so many times to kill Prahlada but by the grace of Lord Vishnu he remained safe at every attempt,no one was able to oppose or kill Hiranyakashipu because he had recieved a boon from Lord Brahma that he would not be killed by any man,woman, or animal, nor through weapon neither in the day nor at night,neither on the ground and nor in the air,in the house or outside.Then to protect his devotee Prahlada,Lord Vishnu took the form of Narasimha (The half man half lion) He dragged the the demon on his lap (Neither ground nor in air) the time was evening (Neither day nor night)and tor his stomach with his claws (No man made weapons) As Lord Krishna said in Gita "Hey Arjuna i come time to time in different incarnations to protect Dharma on this Mother Earth.