Gergerra Rei was a Mech-Lord, possessing a high status as the master of the Kapekan Sect of the Mechanicum's Legio Cybernetica branch. During the Great Crusade in the late 30th Millennium, he led two full cohorts of combat automata (Imperial Robots) which fought alongside the Luna Wolves Space Marine Legion and the Warmaster Horus during many notable campaigns. Like many members of the Cybernetica, Rei eschewed the gross cybernetic augmentations of his colleagues in the Mechanicum in favour of subtle enhancements that did not disfigure or dilute his outwardly human aspect. However, those who knew Rei knew that whatever humanity he did show was rare and fleeting.
He maintained a personal bodyguard that consisted of a maniple of three modified Crusader-class robots that were in his presence at all times. A fourth mechanical, this one custom-built to resemble a female form rendered in polished chrome, walked at his side and served as his aide. No one asked questions about loyalty when Rei was nearby. His machines could hear a whisper among a roaring crowd, and those who dared to suggest aloud that Rei was anything less than the Emperor's obedient servant lived to regret it. Gergerra Rei had been involved in the creation of the Furious Abyss, a unique Imperial Abyss-class Battleship of special configuration, created and built in the Jovian shipyards of Thule by the Renegade Dark Mechanicus faction loyal to Kelbor-Hal, the Fabricator-General of Mars' Mechanicum.
Though he was duplicitous in committing treason against the Imperium, he did so in a way that could not be proven without doubt in a court of law, nor in a way that connected him through any direct means, yet his involvement was enough that the guardians of the Imperium were certain of his treachery. Whatever his crime, perhaps the transfer of certain bribes, the diversion of materials and manpower or the granting of passage to starships that should have been denied, the Kapekan Mech-Lord had done the bidding of the traitor Horus Lupercal. Aboard Saros Station, an orbiting resort platform where the Mechanicum's Jovian elite could find respite and diversion from the works of the shipyards and manufactoria, Gegerra Rei met his ultimate fate at the skillful hands of the assassin known as Koyne, a Shade in the service of the Clade Callidus. Having been a shape-changer for most of his/her life, Koyne often stayed in the form necessary for the completion of the mission and eventually had forgotten his/her original gender. Disguised as a famous female opera singer, Koyne seduced the Mech-Lord and murdered him in cold blood, delivering the Emperor's justice for his betrayal.
- Nemesis (Novel) by James Swallow, pp. 126-127, 129, 132, 133-136, 138