Epimetheus was one of the original founding members of the Grey Knights. He stood before the Emperor alongside the other seven recruits Malcador the Sigillite had selected in the hours before the Siege of Terra and was given command of one of the original brotherhoods.
Epimetheus was orginally of the Dark Angels Legion, serving on Caliban under Luther. How he came to the Grey Knights is unclear. In a conversation with a Daemon Prince of Nurgle, the prince is baffled by Epimetheus's presence as a Grey Knight, telling Epimetheus fate had dictated he be cast to the winds like all the other Fallen. Epimetheus countered that another, a Dark Angel with a most sacred sword(Possibly Cypher), had taken that path in his stead. The timing of this doesn't quiet work out, as the war between Luther and Lion El'Jonson doesn't take place until after the Horus Heresy. However, Epimetheus was already present on Terra with the other recruits and Malcador before the Horus Heresy was over.
Regardless of how Epimetheus became a Grey Knight, the more important fact is he did. According to him, his original progenoid from the Dark Angels was removed and sealed in a timeless capsule beneath Titan along with the geneseeds of all the already implanted recruits Malacdor had. It was replaced with one from an undisclosed source, rumored to be from the Emperor himself.
When Malcador hid Titan from the knowledge of the Traitor Legions as they entered the Sol system, Epimetheus was relatively young and inexperienced. When Titan returned to real-space, he was ancient, older than any of the still living veterans from the Great Crusade and Horus Heresy. Epimetheus would lead a full brotherhood of Grey Knights in the Great Scouring, a war that was many centuries long according to him, rather than the seven years the Legions spent fighting it. Epimetheus would eventually volunteer to stay behind on the planet Pythos to watch over the Damnation Cache. There he spent ten thousand years in suspended animation.
When Abaddon the Despoiler came to unlock the Damnation Cache, Epimetheus awoke once more. He teamed up with a junior Inquisitor of the Ordo Malleus, but refused to take her orders as in his time, the Ordo not only didn't hold authority over Grey Knights, but it was expected the Ordos would be disbanded after they weren't needed.
At the conclusion of the campaign, Epimetheus was take prisoner by Abaddon, who wanted to experiment on the unique gene-seed he still carried and wanted to know where the other still living Grand Masters were.