The rise of Mura
In the age when the devas ruled Swarga in peace, there arose a mighty asura named Mura of the line of Brahma-shapa-born demons. He had won a boon from Brahma that no weapon of any deva could pierce him, and with that boon he defeated Indra's whole army and drove the devas from Swarga. Wandering as beggars through the lokas, the devas came at last to Bhagwan Vishnu on the ocean of milk and cried at his feet.
The thousand-year battle
Vishnu rose from Sheshanag, took up chakra and gada, and met Mura upon the plains of Chandravati. For a thousand years they fought — mountains were levelled, rivers ran backward, the very stars trembled. Yet Mura's Brahma-boon held; no weapon of Vishnu could take his life. Exhausted at last, Vishnu retired to a cave in the northern Himalayas known as Simhavati-guha to rest and think, and there he lay down in yog-nidra.
The birth of Ekadashi
Mura followed him even into the sacred guha and raised his fierce sword to strike the sleeping Vishnu. In that moment, from Vishnu's own body — from the eleven indriyas that are the seat of his shakti — arose a radiant devi with eleven arms bearing eleven weapons, her face blazing like the pralaya sun. She caught Mura's sword-arm in mid-strike, hurled him to the cave floor, and with a single glance from her third eye reduced him to a mound of ash. The devas at the mouth of the guha broke into jai-jai-kar.
The boon
Vishnu awoke, saw what she had done, and rose with a smile that lit the whole cave. He said, 'Devi, you are born of me on the eleventh tithi of Margashirsha Krishna Paksha; you shall be called Ekadashi. Ask any boon.' She folded her hands: 'Prabhu, let whoever fasts on the eleventh tithi of any paksha, in any masa, remembering me and remembering you, be freed of every papa and receive your highest grace.' Vishnu granted it; and thus began the vrat of Ekadashi that has run without break through all the yugas since.
"The originating Ekadashi of the whole year-cycle. Observing this vrat with the katha of Ekadashi Devi's birth is said to be the true dikṣā into a lifelong practice of all 24 Ekadashis, and the beginning of the vrati's own inner victory over the Mura-like desires within."