Lord Vishnu incarnates in various divine forms from time to time for the welfare and protection of creation. The Srimad Bhagavata Mahapurana describes twenty-four principal incarnations of Lord Vishnu. According to the descriptions found in religious texts, among these twenty-four principal incarnations, the fourth is the divine incarnation of Nara-Narayana. Nara and Narayana are two brothers who are both manifestations of Lord Vishnu. In this form, Lord Vishnu appeared on Earth to establish Dharma, i.e., righteousness.

According to the story mentioned in Shiva Purana, Lord Shiva, pleased with one thousand years of intense penance by Nara-Narayana, appeared in front of Them and asked Them to seek a boon. Nara-Narayana, in order to ensure the welfare of all living beings, requested Lord Shiva to reside in the Parthiva Shivalinga that they had created at the place of their penance. Lord Shiva granted Nara-Narayana the boon and agreed to reside at that place forever in the form of Kedareshwara Jyotiralinga.
According to the Puranic story, a demon named Dambodbhava lived in ancient times. He decided to perform intense penance to propitiate Lord Surya. Eventually, Lord Surya, pleased with the demon's penance, appeared in front of him and asked him to seek a boon. Dambodbhava asked Lord Surya to grant him the boon of immortality. However, the deity told him that every living being born in this mortal world would inevitably die one day, thus He could not grant him the boon of immortality. Hence, he must ask for another boon. Upon hearing the words of Lord Surya, that evil demon felt disappointed for a moment, but suddenly a devious idea arose in his mind. He then said to Lord Surya - "O Lord! I request you to grant me the boon of one thousand Kavachas, i.e., armours and each of these armours should be so powerful that only someone who has performed intense penance for one thousand years should be able to pierce even one of the armours. Once the armour is pierced, the person should immediately die afterwards."
Dambodbhava thought to himself that for piercing even one of his armours, his enemy would have to perform penance for one thousand years and even if they succeeded in piercing one, they would die immediately afterward. In this way, he would become immortal. But upon hearing what Dambodbhava desired as a boon, Lord Surya grew concerned, though He was bound to honour His words after making the proclamation. Therefore, Lord Surya said Tathastu and granted the desired boon to Dambodbhava. As soon as the boon was granted, Dambodbhava began wreaking havoc in all directions like a rogue elephant. He started tormenting living beings with his powers. His terror caused uproar in the three worlds. Dambodbhava soon became infamous in the entire universe as Sahasrakavacha, the one with a thousand armours.
Lord Brahma had a mind-born son named Dharma. The name of Dharma's wife was Ruchi. Upon witnessing the atrocities committed by Dambodbhava, Ruchi invoked Lord Vishnu and entreated Him thus - "O Lord! I beseech you, please take birth as my son and destroy this evil demon named Dambodbhava." Lord Vishnu accepted her plea and said - "I shall indeed incarnate as your son to annihilate this evil demon."
In due course of time, Lord Vishnu incarnated in the form of Nara and Narayana from the womb of Ruchi as her twin sons. Immediately after their birth, both brothers showed profound interest in chanting and meditating upon the name of Lord Narayana, because the very purpose of Their birth on Earth was to establish Dharma or righteousness. Therefore, the brothers sought permission from their mother and retired to the forests called Badrivana and Kedarvana, located in the modern-day state of Uttarakhand, to perform intense penance. At present, Badrivana is the location of Shri Badrika Ashrama. Badrinath Dhama is located on the banks of River Alakananda. Two mountain peaks called Nara and Narayana are also located in the same region.
It is said that "Vinashakale Viparitabuddhih", that is, "When the end draws near, intellect fails." In this way, the fate of Dambodbhava led him to Badrinath, the place where Nara-Narayana had been performing intense penance for thousands of years. Intoxicated with the pride of his powers, that unrighteous demon walked into the forest. Suddenly, he found two sages, Nara-Narayana, immersed in deep meditation. That sinful demon began attempting to disrupt the meditation of Nara-Narayana and challenging them for a war. His disruptions caused Lord Nara to open His eyes, but Lord Narayana remained deep in His meditative state. Upon seeing Dambodbhava, Nara instructed him to follow the path of Dharma, but that demon was willing to fight the war. Eventually, Lord Nara empowered a straw with the power of Mantra and transformed it into a weapon and began to fight the demon.
With the power of Lord Nara's penance, one of the thousand armours of Dambodbhava was pierced. However, due to the boon of Lord Surya, Nara died as soon as the armour was pierced. At that very moment, Narayana completed His thousand years of penance and by the virtue of it, acquired the power of Mrita-Sanjivani Vidya. Narayana used the power of Sanjivani Vidya to revive Nara. As soon as Nara was revived, He immersed Himself in deep meditation, while Narayana continued to fight the war with Dambodbhava. Because Narayana had completed His thousand years of penance, He was able to pierce another of demon's armours, but He too died due to the boon of Lord Surya. By that time, Nara had completed His own thousand years of penance, acquired the Mrita-Sanjivani Vidya, and used it to revive His brother Narayana. Upon being revived, Narayana also entered deep meditation.
In this way, the war continued for many thousand years and 999 armours of Dambodbhava were pierced one by one, and only one remained intact. When that sinful demon realised that his end was near, he fled the battlefield and hid himself in Suryaloka, the divine abode of Lord Surya. Narayana followed him to the Suryaloka and requested Lord Surya to surrender Dambodbhava to Him. However, Lord Surya refused to hand over His devotee who sought refuge at His feet. Lord Surya's refusal infuriated Narayana and by taking water in His hand from His divine pitcher, He cursed Surya Deva - "You have protected this demon from suffering the consequences of his actions, so you will take birth with him and suffer the consequences of his actions."
In the due course of time, the combined incarnation of demon Dambodbhava and Lord Surya manifested in the form of Kunti's son, Karna. Being part of Lord Surya, Karna was born with inherent virtues like majesty and generosity. However, he was equally egoistic and evil due to being the part of demon Dambodbhava. Thereafter, Nara and Narayana were incarnated as Arjuna and Lord Krishna respectively and later became the cause of Karna's death. Thus, Lord Vishnu incarnated as Nara-Narayana to destroy the demon Dambodbhava and establish Dharma.
The father of Lord Nara-Narayana is Dharma and their mother is Ruchi. Their grandfather is Lord Brahma. Nara and Narayana both observed Brahmacharya, that is celibacy, throughout Their lives and remained unmarried.
Lord Nara-Narayana are depicted wearing matted locks of hair on head and holding a swan in hands, a discus on feet and the symbol of Srivatsa on chest. Lord Nara-Narayana dons the attire of ascetics.
Moola Mantra -
ॐ नरनारायणाय नमः।
Om Naranarayanaya Namah।