It is the story of Nara-Naryana and Sahasrakavacha. Nara-Narayana were two twin rishis who were an amsa of Lord Vishnu.
According to one story, Arjuna was known as Nara in a previous lifetime in the holy spot of Badrinath in today’s Uttarakhand.
Nara and Narayan were born to Dharma and Murthi Devi.
In their time there lived a demon named Sahasra Kavacha whose body was protected by a thousand armours.
And each armour could only be destroyed by an opponent who had done a thousand years of penance and who could find the demon for a thousand years.
But whoever destroyed armor would immediately fall to the ground and die.
First, Nara meditated and fought with Sahasra Kavacha for a thousand years before dying but only after he had destroyed one of the demon’s armors.
Narayana instantly took his place and resumed fighting with the demon even as Nara was reborn and meditated for the strength to defeat Sahasra Kavacha.
Thus Nara and Narayan alternated until only one armour was left to destroy the demon. With the loss of each armour, the Devil Inside the demon kept decreasing until there was barely any left.
At that point, the demon managed to escape.
It is said that the same demon was later born as Karna, who was slain by Arjuna and Krishna, incarnations of Nara and Narayan.