Aja Ekadashi is also known as Annada Ekadashi.
This was narrated by Lord Krishna to Yudhisthira.
Long time ago there lived a truthful and honest king Harishchandra. While going from Malshej Ghat (Pune to Mumbai) there is a very old historic temple of King Harishchandra. The climb is tough, still local villagers likes to go there on holidays like 15th Aug and others. The fort is too old as per word of mouth. After evening it is advised to leave the mountain (may be due to some foxes or jackals or something else)
Now once Saint Vashisht praised Harishchandra and said he is a very truthful man. The myth that Truth can be observed only by Heavenly Gods and not humans is turning false. King Vishwamitra now a saint disagrees and decides to test him.
As per other story Saint Vishwamitra was annoyed with God of Heaven Indra as He claimed that humans are too low to observe and follow Truth. Hence He picks Harishchandra and tests him and shows to the world that humans are also capable to adhere Truth.
Harishchandra is tested so heavily that he loses his kingdom for sake of truth. Later his wife is sold as slave but he does not give up truth. Then his son dies but he was a slave of a person who had given him job to burn dead corpses. So when his son dies , and his wife also arrives weeping, he feels like embracing the child's corpse and weep and give sympathy to his wife.
But being a slave he holds to Truth. He remains firm. He was not supposed to act like a father. He bears pain and starts the duty of burning corpse of his son. So heavy is his pain that the universe is imbalanced and the Trinity - Creator, Preserver and Destroyer had to interfere. Everyone accepts the Truth that humans are capable to adhere Truth and the dreadful test stops there.
It is said when in distress and pain King Harishchandra asks Gautam Rishi the Saint to give some remedy over his miserable condition. Saint Gautam advises him to practice or do Aja Ekadashi. Due to piety of this Aja Ekadashi all sins got burnt. The ego strongly holded by others regarding inability of humans to adhere Truth was melted away. The tendency to Test was weakened. Humanity, pity and other good feelings started arising in psychic fields of surrounding superior and inferior powers. He got his kingdom and wife back and his dead son became alive. At end of his life, he with his whole family and also the people of his kingdom enters the eternal heavens.
This was the impact of Aja Ekadashi. It is like a flame burning sins.
Advantages of Ekadashi
1) Souls in afterlife of our parents, grandparents and their grandparents if stucked in hell or Pitar Lok are greatly relieved
2) When divine souls are born, the musical instruments of heaven starts creating music automatically. That way this fast on Ekadashi has potential to bloom lotuses in astral body and lotuses in other world.
3) When we sacrifice, others get. It is a piety.
4) This fast has power to dispel distress and panic conditions.
5) Because of remaining hungry, we remain weak hence any negative energy matrix (curse, grudge, ill will etc.)programmed to doom us weakens as we do not fight back but tolerate and the bad time passes away. Opposition strengths the evil and tolerance makes evil alone, panics the evil. Either that evil gives up those evil ways or gets doomed when the pot of its sins gets filled, by the virtue of Karma.
6) This Vrata empowers Lord Vishnu with a reason to help you. Being impartial He is supposed to be equal but this vrata gives Him authority to help you, if at all, He feels His interference can make your life more better. Eg. Bali - the master of 3 worlds was ruined by Vaman. Source was just a fast done by Aditi - the mother of heavenly gods. She fasted for Lord Vishnu and prays her to make the demons fall from heaven and her sons - the heavenly gods get back their heavens.
Lord Vishnu got a reason to interfere and He popped in.