TALES OF NAR-NARAYAN

To Know list of all avatars of Lord Vishnu Please read this post
Fourth incarnation of Vishnu is Nara and Narayana.  They are twin brothers born to Dharma Deva and his wife Murti (Narayaneeyam 16.2).  Both of them killed a demon after performing tapas for many years.  They taught spirituality and won over Cupid, the God of love.

MANIFESTATION OF NAR-NARAYAN
Sage Pulastya says- "Dharma, the possessor of divine body and who manifested from the heart of Lord Brahma married Murti, the daughter of Daksha. Four sons were born to them- Hari, Krishna, Nara and Narayan. Hari and Krishna engaged themselves in the practice of Yoga while
Nar and Narayan went to the Himalaya Mountain and started doing penance for the welfare of humans.
Indra became frightened by their arduous penance. He feared that he might lose his authority and power. He sent beautiful Apsaras to disturb their penance. He himself went to that site accompanied by Kamadev and Vasant (spring).

PRAHLADA MEETS NAR-NARAYAN
Once Narayan saw all three of them coming and he treated all of them with respect. As he was aware of the reasons for their arrival, he decided to give them a surprise. He made the figure of a beautiful woman on his thigh with a flower. The next moment, the figure transformed into a live woman, which surprised all of them.
For Kamadev, the beautiful woman resembled his wife Rati. He became so infatuated by her beauty that he forgot about the actual purpose of his visit. The same thing happened to Vasant and all the Apsaras. Everybody who had come with the desire of victory had actually become a victim.
Seeing all of them surprised and amazed, Narayan told them that the beautiful woman was created by him from his thigh. He also instructed them to take away that beautiful woman to heaven. Kamadev and Vasant were captivated by Narayan's power. They also became terrified. They went back to heaven accompanied by Urvashi and narrated the whole story to Indra. Even Indra was amazed. In a short time, the amazing deeds of Narayan spread in all the directions and the various lokas.
Once, Prahlada- the son of demon King Hiranyakashipu asked sage Chyavan about the most revered and sacred place of pilgrimage in the whole world. Sage Chyavan told him about Naimisharanya- the most sacred place of pilgrimage on the earth. Prahlada visited Naimisharanya accompanied by many other demons. After reaching there, all of them took bath in a river. While roaming about in the city, Prahlada saw the trunk of a tree pierced with numerous arrows. Not far from the tree, he saw two hermits doing penance. Two divine bows and a pair of quiver were lying by the side.
Prahlada was amazed as to why the hermits would require weapons. He revealed his thoughts to the hermits. Both the hermits replied that a person who works according to his capabilities, earns respect. Prahlada became angry at their reply. He asked them as to what were their capabilities. Actually, the hermits were none other than Nar and Narayan. Both of them told Prahlada that they were invincible and nobody could defeat them in battle. Prahlada's anger crossed all limits. A tremendous battle ensued between Prahlada and Nar-Narayan.

PRAHLADA BLESSED BY NAR-NARAYAN
A ferocious battle was fought between Prahlada and Nar-Narayan in which all types of lethal weapons were used. The battle continued for 1000 divine years. Ultimately, Prahlada was
defeated in this battle. He went to Vaikunth and asked Lord Vishnu as to why Nar-Narayan was invincible. Lord Vishnu revealed to them that Narayan was his own incarnation. He also told them that Narayan could be won only by devotion and not by power.
Prahlada decided to relinquish his throne and appointed Andhak as his successor. He went to Badrikashrama and made salutations to Narayan in reverence. Narayan was surprised at the sudden change in his behaviour. He asked Prahlada as to what the matter was. Prahlada then said- "Who can defeat you? You are none other than Lord Janardan. You are the one who has taken incarnation of Hrishikesh, Chakrapani and Hayagreev."
Narayan was pleased by his devotion. He told Narayan that though he was not able to defeat him in the battle but he was successful in doing so by his devotion. Narayan also blessed Prahlada. Prahlada then returned to his capital. Andhak expressed his desire to make Prahlada the king once again but it was turned down by him. Prahlada spent the rest of his life preaching on the virtues of religiousness and truthfulness.