According the RedEye, there have been ten different versions told by ten different guys of the Legend of Atlas. It's unknown where he came from but there three different versions of his origin. The first is that he was a soldier from the Great War and took a direct hit from a nuclear bomb, simply shrugging it off. The second is that he appeared out of thin air, as if conjured up by the wasteland itself. The third is that he was offspring of a super mutant, deathclaw, and the Devil.
The first story of Atlas involved him traveling somewhere south of Boston and single-handedly killing a whole band of super mutants, in which he bit the head off of their leader's dog.
In another story, he took a group of five men somewhere in downtown Boston to confront a rival gang, which ended with Atlas walking out with the leader's son because he said mean things about Atlas's mother.
In his final story, Atlas traveled to a town far north of Boston called Andover. The town was inhabited by a gang of raider known as the Junkers, who's leader was blonde woman named Hope. He fell in love with Hope and wanted her to surrender to him, so Atlas gathered an army of over 200 men to intimidate her into surrendering. Hope denied his first attempt to court her, so he then convinced a few of his lieutenants to light themselves on fire in order to show the loyalty of his men. This too is ineffective and when Hope denies him for a second time he is infuriated, causing him to smash his giant fists into the ground. The shockwave of the blast shattered windows in Diamond City, caused a whole section of Lexington to come crashing down, and leveled everything surrounding the town of Andover. The next day, he gave his final warning to Hope, in which he said he will storm the town if she does not surrender. This finally caused Hope to give into his demands and raise the white flag. She then walked out the front gate and convinced Atlas to shake on the deal. While shaking hands, she detonated two Mini nukes that she had strapped to herself, killing both herself and Atlas in a massive fiery explosion that blinded men hundreds of miles away and caused birds to drop from the sky for days. They never found all of Atlas's remains.