In Satya Yuga, there was an awful Demon named Mura. Mura was extremely powerful and awful. With his extraordinary powers Mura defeated not only God Indra but also many other Gods and stared ruling Indra Loka. To end the tyranny of Mura all Gods went to Lord Shiva to seek some help. Lord Shiva suggested them to approach Lord Vishnu, the master of all three Lokas.
Lord Vishnu decided to defeat Demon Mura to help Gods. Lord Vishnu and Gods reached Chandravati, the ruling city of Mura. Lord Vishnu was at one side and Demon Mura with all his army was at the other side. As story goes, Lord Vishnu annihilated whole army of Mura with His Sudarshan Chakra and the divine Gada but all His weaponry proved ineffective in front of Demon Mura power. Neither His Sudarshan Chakra nor His Gada was able to cut the head and break the neck of Demon Mura.
The prolonged battle converted into Malla Yuddha (मल्लयुद्ध), one to one fight without any weaponry. It is believed that the fight between Lord Vishnu and Mura continued for 10,000 years. On seeing no end to this endless battle Lord Vishnu stopped fighting and went to Badrikashram to take rest at Hemvati Caves.
Demon Mura chased Lord Vishnu and reached Badrikashram. Demon Mura found Lord Vishnu sleeping and thought it to be a great opportunity to kill Lord Vishnu. To counter attack such a devilish feeling inside Demon Mura, a powerful girl was born out of divine body of Lord Vishnu. The girl was glorious, powerful and adorned with all sorts of weaponry to protect Lord Vishnu. This feminine power of Lord Vishnu defeated Mura in the fight and killed him after putting Her leg on his chest and cutting his head.
All Gods started praising the girl for Her extraordinary power. When Lord Vishnu awoke, He was unaware about His own power and insisted the girl to introduce herself. The girl introduced herself as the luster of Lord Vishnu itself who was born out of Yoga Maya of Lord Vishnu. Lord Vishnu was extremely pleased with His feminine power and asked Her to seek any reward in return for the great work.
The girl asked "Please bestow me the power so that anyone who observes the fasting for me would get rid of all sort of sins and get salvation. Please bless me so that I become the most significant among all pilgrimages and able to bestow Dharma, Dhanya and Moksha to my devotees."
Lord Vishnu blessed the girl and told "You would be known as Ekadashi as you were born on Ekadashi Tithi. You would be worshipped in all Yugas not only by humans but also by Gods. Nothing else would please me more than Ekadashi fasting. Your devotees will enjoy all sorts of worldly pleasure and would get salvation at the end of their life."