Maxi-chlorians represent an invention of Darth Tenebrous, conceived as a method for attaining a kind of immortality. Throughout his existence, Tenebrous administered a retrovirus to himself, which then contaminated a portion of his midi-chlorians, transforming them into maxi-chlorians, but at the cost of his powers of precognition. These maxi-chlorians possessed an extended lifespan compared to normal midi-chlorians and were designed to persist following Tenebrous's demise, instead of returning to The Force.
Upon his death, these altered midi-chlorians endured, carrying Tenebrous's awareness. They departed from his deceased physical form and entered his apprentice, Darth Plagueis, effectively providing Tenebrous with a "vessel" and granting him access to his apprentice's capacity for foresight. Nevertheless, upon recognizing that Plagueis's eventual downfall would undermine his schemes, Tenebrous became alarmed and ejected himself from Plagueis's body, where he was condemned to re-experience his last moments repeatedly, apparently for an infinite duration, unless the maxi-chlorians were somehow destroyed.