The Omega Vault is a sealed series of ancient engines, catacombs, and chambers that sits at the heart of the Deathwatch's Watch Fortress Erioch in the Jericho Reach. The Omega Vault has been part of the Watch Fortress since it was established in the Jericho Reach, though what it contains at its centre remains unknown to any now living. The vault itself is a broadly cylindrical portion of the Watch Fortress that spans many levels running through its core. There is only one entrance into the Omega Vault; a single door forty paces high. The door is sealed and armoured by layered Void Shields and Adamantine plating as thick as a warship’s prow. Engraved upon it are the symbols of the Deathwatch and the elder Terran rune "Omega," surrounding an image of the Emperor strangling a coiling serpent. Above these great doors are words in High Gothic that read, “And The Last Fortress Is Truth.” Each Battle-Brother freshly stationed at the Watch Fortress is brought before these words to take his Oath of Vigil.
No living being can open the doors of the Vault; they open themselves at the command of mysterious ancient devices at the heart of the Vault itself. Those who have been inside pass along corridors and down spiralling stairs lined with catacomb subvaults, each barred with its own blast door and surrounded with dust-dry parchment fragments and archaic marks etched in silver. Through each door awaits a secret of the Omega Vault held in an armoured chamber, each protected by powerful automated defences and locks of unfathomable complexity. The Omega Vault is known to yield up a portion of its cryptic artefacts and information in direct response to events unfolding within the Jericho Reach. There is also evidence that the central data-reservoir of Watch Fortress Erioch is bound to something deep within the Omega Vault. The Vault has, on occasion, responded to new information sent in from Watch Stations or operational reports from Deathwatch missions by revealing a portion of what was placed within it many millennia ago.
However, this process remains fraught with mystery, particularly as to whether the Vault’s actions are based entirely on a pre-ordained sequence or are at least partly reactive in nature. Perhaps most tellingly, there are numerous events of great import that have transpired in the Jericho Reach and failed to trigger any response from the Vault. Most recently, for example, there was the incursion of the Tyranid threat into the Orpheus Salient. So far, this event has drawn no response from the Vault, although specific dangers since, caused by the Tyranids, have triggered activity within the Omega Vault. It is an enigma that continues to trouble the thoughts of Deathwatch Space Marine and Inquisitors alike. Only one chamber has ever been discovered and unlocked each time the Omega Vault has opened itself. Once whatever waits in the open chamber has been removed, the Vault seals itself again, sounding a sonorous bell to warn of its closure and of the fatal consequences that follow should anyone tarry overlong.
In the past, the Vault has provided tomes of archaic lore, strange fragments of alien devices, ancient weapons forged in times long gone, world-slaying virus canisters, enigmatic star charts, and bizarre xenological specimens. On one occasion, its gift was an ancient Astropath held in a stasis field. When awoken, the Astropath sent one brief astropathic message and spoke two words before expiring from extreme old age, his final duty discharged. At the centre of the Omega Vault lies another, inner vault: a secret of secrets, bound with scores of locking mechanisms which have slowly opened themselves, one-by-one, down the centuries. The Chamber of Vigilance believes that sealed within this chamber is the final truth and, perhaps, the last weapon the Deathwatch will have need for in the Jericho Reach.
What plan or pattern ordained in the ancient past may have necessitated the construction of the Omega Vault — and what hands constructed it—remains a mystery many have pondered. Further riddles are posed by the seemingly precognitive abilities and resources required to create such a Vault. What is without doubt is that the creators of the Omega Vault were human, a truth evident not only from the elements of its design, but a further significant detail. Contained within the Vault’s air is an appallingly powerful synthetic pathogen that is entirely inert outside the Omega Vault’s walls. This pathogen is an ancient and un-replicable weapon, utterly lethal to all kinds of life save one: Mankind. What is certain is that the Watch Fortress' Chamber of Vigilance has a duty to protect the Omega Vault and to use and follow what information it yields in the Imperium’s defence as they judge fit. It is a duty they have had to discharge with greater frequency in the decades since the Jericho Reach's Warp Gate opened and the beginning of the Achilus Crusade in 777.M41. Now, only a handful of locks remain closed on the innermost vault.
- Deathwatch: Core Rulebook (RPG), pp. 328-333