Lord Revenant Vercingetorix is one of the oldest living members of the Imperium. He was a Captain in the Iron Hands in the days of the Emperor's great crusade. Months before the Istvaan massacre, Vercingetorix was swallowed whole by a giant Squiggoth. Assuming he was doomed to death, Vercingetorix began igniting and placing all his explosives, filling the creature's mouth and gullet with primed meltabombs. He killed the creature, and the Legion's apothecaries rescued his shatterd, mutilated body, intending to inter him in the Legion's crypt - only to find, to their astonishment, that he was still alive. The Primarch decided to honour his sacrifice with the greatest reward of all - Vercingetorix was placed in a sarcophagus and interred in a Contemptor-pattern dreadnought. The Captain's remains were taken back to an Iron Hands monastery in the Kokytus system deep in the Segmentum Pacificus to be prepared.
When the Horus Heresy broke out, all the Iron Hands in the outpost were called to war. They left the monastery's servitors with orders to care for Vercingetorix while he convalesced, and to awake him when the sons of Ferrus Mannus returned. When the Legion was all but destroyed by Horus's treachery on Istvaan, the monastery was forgotten. Its servitors continued to mindlessly carry out their orders, and for thousands of years, Vercingetorix slept, dreaming dreams of conquest. After the siege of Terra, billions of Administratum records were left in shambles or lost, and the Imperium slowly forgot of the existence of the Iron Hands' fortress, and eventually of Kokytus itself. So it was that the relics of the past sat, unused, gathering dust, until Warmaster Alligheri's forces made planetfall on Lucifer in the 34th millennium.
The explorators who charted Lucifer discovered a bounty of technology dating as far back as the Golden Age of Man, and perhaps even from the Dark Age of Technology. Most of the relics were impossible to salvage thanks to the planet's harsh climate and the scavenging locals, but enough functional pieces were found to bring the attention of the Cult Mechanicus. It was one of their Magoi who discovered the gate. The gate was buried in a cave deep in the tall mountains of the planet, in a cave full of the ruins of ancient machines. A massive construction, thirty meters wide and twice that in height, the door was incredibly strong and resisted all efforts to breach it - from plasma cutters to siege drills to melta torches. It was adorned with symbols of Mars and of the Imperium, and in the middle of the door was the silver gauntlet of the Iron Hands. Though the Magos had hoped to open the door himself and claim whatever technological prizes might lay beyond, he finally relented and called for the Steel Revenants to seek their aid in unraveling the mysteries of the door.
Nonoe was more surprised than the Magos when, as soon as the Revenants' Iron Father placed a hand on the door, ancient gears sprang to life and the door began so open, sliding back into the walls of the cave on either side. Beyond was what could only be described as a treasure trove of Space Marine relics. Though during the Great Crusade the Iron Hands' abandoned outpost was considered a modest one, it was enormous by the somewhat impoverished standards of the 34th millennium. Each room contained more wonders: racks of boltguns, stands of power armour and even dozens of tactical dreadnought suits; archives full of after-action reports, officer's logs, and even rituals from the days of the Great Crusade; dozens of Rhinos and battle tanks; even some fully-functional servitors mindlessly carrying out tasks even after four thousand years. But the greatest prize was in the fortress's forge room: A massive stockpile of bionic parts and three Contemptor dreadnought bodies. And, most astonishing, one of the Contemptors held a still-living warrior within. So it came to pass that, for the first time in four thousand years, Vercingetorix awoke.