The mythological story of Prahlad, is described in Vishnu Puran as the fourth incarnation of Vishnu, Nara-simhavatar.
Soon Hiranyakashipu got a son. He was named Prahlad. Prahlad grew up to be a lovely boy. His father was unaware of the fact that Prahlad was an ardent devotee of Vishnu. Hiranyakashipu, however, wanted his son to be the king of the Asuras, strong and ruthless. One day Hiranyakashipu lovingly asked his son, "Dear Son, what do you think is the best thing in life?" Prahlad replied, "To renounce the world and seek refuge in Vishnu." Hiranyakashipu was shocked to hear this. He could not tolerate the idea that his son loves the sworn enemy of the Asuras, Lord Vishnu. He felt that someone deliberately trying to brainwash his son, undermining his future dreams. To find this out, Hiranyakashipu sent Prahlad to his teacher.
The teacher lovingly asked Prahlad, "Who has taught you about Vishnu?" Prahlad calmly replied, "Vishnu himself." The teacher was furious. He called for his cane and beat Prahlad mercilessly. Prahlad stayed quiet and completed his education. He quietly shared his feelings with his friends without the knowledge of his teacher. His serene behavior and thoughts of divine happiness appealed the young students. They believed that Vishnu is all prevailing, caring and the true preserver of the universe.
After his graduation, Prahlad returned to his father. Hiranyakashipu expected his son to behave like an Asura prince. So he loving asked, "What worthwhile occupation did you learn from the teacher?"
Prahlad unhesitatingly said, "Father, the only worthwhile occupation that I learnt is to dedicate my life in the work of Vishnu, the Lord of Preservation."
Hiranyakashipu shouted, "You wicked teacher, you taught my son to worship my worst enemy, Vishnu!" The teacher repeatedly tried to assure that he did not teach such a thing.
Prahlad tried to convince his father "Father, my friends in the school believed me when I said Lord Vishnu reveals Himself to all who devoted to Him. You should also do the same and humbly accept Vishnu as the Lord of the Universe."
Hiranyakashipu raged in anger and ordered his subordinates to kill Prahlad. They tried different means but Prahlad stayed in deep meditation of Vishnu and nothing hurt him.
Hiranyakashipu became thoughtful. "This boy is immortal and fears nothing. Will he be the cause of my death?" He shouted in anger, "You wicked wretch! Are you trying to destroy me and my race! The rulers of the three worlds quake before me. What is the source of your power?"
"Oh father" replied Prahlad, "It is Vishnu. He is the source of our power. He is present everywhere and protects all who believed in Him."
Impatient Hiranyakashipu could not bear any longer. He pulled out his sword and challenged Prahlad, "I will see how Vishnu can protect you." Hiranyakashipu pointed towards the pillar outside the hall and shouted, "Is He present in this pillar?"
"Yes father" was the calm reply from little Prahlad. Hiranyakashipu outrageously ran towards the pillar shouting, "I will first kick the pillar to prove that you are wrong and then I will kill you."
When Hiranyakashipu kicked the pillar with great force, to his utter surprise, he saw the pillar burst open and Lord Vishnu emerged as Nara-simha, half human and half lion. Hiranyakashipu remembered his first death wish, "To be killed by some one who is neither human nor beast." It was also the twilight hour, neither day nor night, the second death wish. Nara-simha caught Hiranyakashipu and carried to the threshold of the courtroom. The place was neither inside nor outside. This was Hiranyakashipu’s third death wish. There Hiranyakashipu was killed by the fourth incarnation of Vishnu, Nara-simha. Other three were fish, turtle and wild boar. After killing Hiranyakashipu, Narasimha sat at his thrown and peace came back to the earth.
All the gods came to pay homage to the incarnation of Vishnu and asked Prahlad, who was not the least afraid of the horrible beast, to pacify him so that He comes back to his normal peaceful form. Narsimha was touched by Prahlad’s devotion. He came back to his normal form and asked Prahlad to ask for any boon. Prahlad humbly said, "Let my father be purified of all his sins." Vishnu blessed Prahlad and asked him to reign in his place and perform the duties of a just ruler. Prahlad ruled wisely and well ever after.