Story of Nrsimha Dev!

tava kara-kamala-vare nakham adbhuta-sringam

kesava dhrita-narahari-rupa jaya jagadisa hare

O Kesava! O Lord of the universe! O Lord Hari, who have assumed the form of half-man, half-lion! All glories to You! Just as one can easily crush a wasp between one’s fingernails, so in the same way the body of the wasplike demon Hiranyakasipu has been ripped apart by the wonderful pointed nails on Your beautiful lotus hands. (Sri Dasavatara-stotra)

Nrsimha dev is the fourth incarnation of Lord Krishna. He appeared as a half-man, half-lion to protect his dear devotee, Pralada from his demonic father Hiranyakashipu. Below shortly describes this story.

The Curse of the Four Kumaras

After having travelled all over the material creation, the four sages Sanaka, Sanatana, Sanandana and Sanat-kumara, entered the spiritual sky, wherein the Vaikuntha planets are situated. They were able to do this due to being freed from all material contamination. They easily passed through six gates. 

The sages, who had nothing to cover their bodies but the atmosphere, looked just like five-year-Jaiya-Vijayaold boys, although they were the oldest created beings, next to Brahma. However at the seventh gate the two gatekeepers, Jaya and Vijaya rudely blocked the Kumaras’ entrance with their staffs upon seeing them. The Kumaras were never to be prohibited from entering Vaikuntha. 

Angry, the sages placed a curse on the two guards. When the gatekeepers realized that they were being cursed by the brahmanas, they at once became very afraid. After falling down at the Kumaras’ feet, they begged for mercy. 

The Supreme Personality of Godhead, having learned of this incident, came there by foot accompanied by the goddess of fortune.  The Kumaras realized that it had been improper for them to curse the doorkeepers, in spite of the offense that had been committed.

Being too late as the guards were already cursed, Jaya and Vijaya was given two choices: repeat the cycle of birth and death in the material world as a devotee for seven lifetimes or as a demon for three lifetimes. 

They choose three lifetimes of being born as demons for it will be quicker for them to return back to Godhead. 

Their first birth was as Hiranyaska and Hiranyakashipu.

Hiranyakashipu’s anger

After hearing the news of his brother’s death, Hiranyaska by Lord Krishna’s boar avatar, Varaha. Hiranyakashipu was in anger and wish to seek revenge on the Lord of the Universe.

The Boon

He performed severe austerities to get the attention of Brahma, and asked to be granted immortality. Brahma was himself a mortal being and so could not grant him immortality. But he did give Hiranyakashipu the boons that he could not be killed by any man or animal, by anything living or dead, during the day or night, inside or outside, on the land or in the sea or sky.Lord Brahma Sprinkles Spiritual Water on Hiranyakashipu's Body

Hiranyakashipu Has Prahlada Thrown from a CliffAppearance of Nrisimhadev 

Hiranyakashipu tormented his youngest son Pralada, a pure devotee of the Lord for worshiping his enemy and not following the demonic education. Lord Krishna always protected the young boy.

One day, Hiranyakashipu asked his son about Lord Krishna’s whereabouts. Pralada replied that He is everywhere. He asked if he was in the pillar. Pralada replied that He is everywhere. In anger Hiranyakashipu took his sword and as he was about to cut down the pillar, Lord Krishna burst out from the pillar as Nrisimhadeva.

By killing Hiranyakashipu in the form of a half-man and half-lion, with his fingernails, at dusk, in the threshold of the palace and on his lap, Nrisimhadeva simultaneously respected the boons given by Brahma and saved His dear devotee Pralada Maharaja.