Lord Narsimha is the 4th Incarnation (Avatar) of Lord Vishnu and is supposed to be the most fierce avatar as per Ancient Vedic Texts and Scriptures. Lord Vishnu took this avatar to destroy the demon king Hiranyakashyapu who was determined to stop anybody from worshipping any Gods including Lord Vishnu. Demon King Hiranyakashyap was granted a boon by Lord Brahma due to which he literally became immortal after which he declared that only he should be worshipped instead of Gods. All the people living in his kingdom followed his order but his own son prahlad declined to accept this since he was an ardent devotee of Lord Vishnu. This enraged Hiranyakashyap to the extent that once he tried to kill Prahlad with his own sword and at that time, Lord Narsimha emerged out of a Pillar roaring as his upper half was the form of a Lion and killed Demon King Hiranyakasyap with the Nails of his bare hands by keeping the Demon King on his lap so that the boon granted by Lord Brahma was honoured as well as the Demon  King was slayed.