Oathkeeper, its full name recorded as Oathkeeper of the Twenty Ninth Intercession, is the name of a mysterious alien Silicate imprisoned within the Deathwatch's Watch Fortress Erioch in the Jericho Reach.
The rare and secretive species of intelligent xenos known as Silicates are all but unknown on the eastern rim of the galaxy. It is believed that their homeworlds lay far coreward, and that the disaster that overtook them has yet to push their surviving starships so far rimward.
Nonetheless, a being purported to be one of their kind is held in the Xenos Bestiarium of Watch Fortress Erioch -- an egg-shaped silicate pillar over two metres tall. On close inspection, the vaguest outlines of a humanoid shape can be determined on the surface.
The xenos is only able to communicate telepathically and few choose to make a connection with its slow, alien thought processes. Most will not speak of the experience, and those that do seem deeply troubled by it.
The being has declared itself to be "Oathkeeper of the Twenty Ninth Intercession" to those that have deigned to commune with it. The Oathkeeper's fractile, multi-layered meanings are hard to interpret but it appears to consider itself a hostage voluntarily given by its people as surety for an agreement struck long ago.
The Omega Vault appears to feature strongly in its thoughts, and secrets pertaining to its construction and ultimate purpose always seem to lurk just beyond reach.
The Oathkeeper itself seems incapable of movement but it has been noted to tremble slightly before each opening of the Vault. Records show that on two occasions the Oathkeeper has mentally broadcast what appeared to be warnings in an unknown tongue with surprising power. The cell's psychic hexagrammic wardings have been strengthened after each of these events.
Precisely what agreement the Oathkeeper stands surety against is now lost in the mists of time, as is the date when it was first brought aboard the fortress. Data probes have been sent into the catacombs on several occasions and frequent cogitator searches have been made but to date little has been found in relation to either the Silicates or the Oathkeeper.
Most intriguingly, an inventory of the Xenos Bestiarium dating from the 39th Millennium lists a "Slow-moving Silicate being, 1 m tall" in the holding cell now occupied by the Oathkeeper.
Testing indicates the being is at least partly artificial and heavily engineered at a molecular level. Apothecary Winslas has postulated it to be a symbiotic grafting of a carbon-based lifeform with a self-replicating silicate matrix, perhaps intended as an exotic form of data storage that ultimately gained its own form of sentience.
Adepts of the Imperium believe that the being has been in a transitory state over the course of its incarceration, slowly accreting mass and only becoming entirely sessile over the last few centuries. Following the Jaxxon Principle, this will likely lead to an expansion of its mental powers and abilities over time.
Certain resonant frequencies have been found to be capable of shattering samples taken from the silicate entity. A battery of high-powered harmonic emitters has been built into the walls of the cell containing the Oathkeeper so that it can be destroyed with the flick of a switch if the need arises.
- Deathwatch: Rites of Battle (RPG), pg. 242