Narasimha

Bhagavata Puran describes that in his previous avatar as Varaha, Vishnu killed the raksha Hiranyaksha. The brother of Hiranyaksha, Hiranyakashipu, wanted revenge on Vishnu and his followers. He undertook many years of austere penance to take revenge on Vishnu: Brahma thus offers the demon a boon and Hiranyakashipu asks for immortality. Brahma tells him this is not possible, but that he could bind the death of Hiranyakashipu with conditions.

Hiranyakashipu agreed:

O my lord, O best of the givers of benediction, if you will kindly grant me the benediction I desire, please let me not meet death from any of the living entities created by you. Grant me that I not die within any residence or outside any residence, during the daytime or at night, nor on the ground or in the sky. Grant me that my death not be brought about by any weapon, nor by any human being or animal. Grant me that I not meet death from any entity, living or nonliving created by you. Grant me, further, that I not be killed by any demigod or demon or by any great snake from the lower planets. Since no one can kill you in the battlefield, you have no competitor. Therefore, grant me the benediction that I too may have no rival. Give me sole lordship over all the living entities and presiding deities, and give me all the glories obtained by that position. Furthermore, give me all the mystic powers attained by long austerities and the practice of yoga, for these cannot be lost at any time. Brahma said, "Tatha asthu" (be it so) and vanished. Hiranyakashipu was happy thinking that he had won over death.

One day while Hiranyakashipu performed austerities at Mandaracala Mountain, his home was attacked by Indra and the other devatas. At this point the divine sage Narad intervenes to protect Kayadu, whom he describes as 'sinless'. Following this event, Narad takes Kayadu into his care and while under the guidance of Narad, her unborn child (Hiranyakashipu's son) Prahlad, becomes affected by the transcendental instructions of the sage even at such a young stage of development. Thus, Prahlad later begins to show symptoms of this earlier training by Narad, gradually becoming recognised as a devoted follower of Vishnu, much to his father's disappointment.

Hiranyakashipu furious at the devotion of his son to Vishnu, as the god had killed his brother. Finally, he decides to commit filicide.] but each time he attempts to kill the boy, Prahlad is protected by Vishnu's mystical power. When asked, Prahlad refuses to acknowledge his father as the supreme lord of the universe and claims that Vishnu is all-pervading and omnipresent. Hiranyakashipu points to a nearby pillar and asks if 'his Vishnu' is in it:

"O most unfortunate Prahlad, you have always described a supreme being other than me, a supreme being who is above everything, who is the controller of everyone, and who is all-pervading. But where is He? If He is everywhere, then why is He not present before me in this pillar?"

Prahlad then answers, He was, He is and He will be. In an alternate version of the story, Prahlad answers, He is in pillars, and he is in the smallest twig.

Hiranyakashipu, unable to control his anger, smashes the pillar with his mace, and following a tumultuous sound, Vishnu in the form of Narasimha appears from it and moves to attack Hiranyakashipu. in defence of Prahlad. In order to kill Hiranyakashipu and not upset the boon given by Brahma, the form of Narasimha is chosen. Hiranyakashipu can not be killed by human, deva or animal. Narasimha is neither one of these as he is a form of Vishnu incarnate as a part-human, part-animal. He comes upon Hiranyakashipu at twilight (when it is neither day nor night) on the threshold of a courtyard (neither indoors nor out), and puts the demon on his thighs (neither earth nor space). Using his sharp fingernails (neither animate nor inanimate) as weapons, he disembowels and kills the demon. Kurma Puran describes the preceding battle between the Purusha and demonic forces in which he escapes a powerful weapon called Pashupata and it describes how Prahlad's brothers headed by Anuhrada and thousands of other demons "were led to the valley of death (yamalayam) by the lion produced from the body of man-lion" avatar.

Before parting, Narasimha rewards the wise Prahlad by crowning him as the king.

--extracted from the Wikipedia entry.