The fourth of incarnation of Vishnu dashavatara is Narsingh avatar (half man half lion)
The story of Hiranyakashyap and Brahma’s boon
Hiranyakashyap was a demon king who did severe penance of Brahma. Brahma became pleased with him and asked for a boon. Hiranyakashyap demanded immortality. Brahma replied that this is not possible as everyone who takes birth has to die, so ask something else.
Hiranyakashyap then asks “no one created by you can kill me; I cannot die inside the house or outside, during the day or the night. I cannot be killed by any weapon by any human or beast, living or non living, any demi-god or demon. ” Brahma granted him this boon.
His son Prahlad
Thus Hiranyakashyap became very strong and he started asking people to treat him as god. But his own son, Prahlad was a great devotee of Lord Vishnu. He was initiated by Sage Narada and he belived in none other than Narayan. Knowing this, Hiranyakashyap became very angry and tried to dissuade Prahlad from worshipping Vishnu.
He wants to kill Prahlad
He tried to kill him by burning him in fire, by giving him poison, by crushing him under elephants’ feet, by rolling down from high hills, by snake bites, by many kinds of weapons, but because of his devotion, Prahalad could not be killed by any means.
Hirankashyap’s sister Holika, offered a solution. She sat with Prahalad on a pyre, thinking that the flames would burn Prahalad and she would be saved by a cloth which was immune from fire. But by god’s grace, Holika was burnt and Prahalad was saved. This is still celebrated as Holika dahan which is followed by Holi, a prominent Indian festival.
One day, Hiranyakashyap challenged Prahalad that where is his god. Prahalad replied – he is everywhere. Out of rage, Hiranyakashyap asked, is he in the pillar and broke the pillar with his mace.
Emergence of Narsingh avatar
Duly, Narsingh avatar or fourth dashavatara of lord Vishnu appeared in form of half lion and half man. It was twilight (neither day nor night) and it was the entrance of the palace (neither inside nor outside). At that moment the Narsingh Avatar ( neither man nor beast), killed Hiranyakashyap with his nails. (Not with a weapon).
Note: Hiranyaksha, brother of Hiranyakashyap, was killed by Varaha avatar of Dashavatara and Prahlad grandson Bali was neutralized by another dashavatara (Vamana) of Lord Vishnu.