They are charged with the vitally important task of standing eternal vigil over the Dark Cells, where terrible entities and artefacts are locked away deep beneath the Imperial Palace on Terra, eldritch terrors from the depths of Old Night that could annihilate the Imperium.
Terrors of Old Night
Many human mythologies, both ancient and the product of the collapse of civilisation on thousands of worlds during the Age of Strife, carry legends of the apocalyptic wars of the gods in whose aftermath terrible weapons are left upon the desolate battlefield for the gods' inheritors to find.
At the height of humanity's ancient dominion during the Age of Technology, unbridled and recklessly potent human science had unlocked myriad potentials within the human genome as well as releasing forbidden technological terrors upon humanity as it suited the whims of the scientist-kings of that lost age. Nowhere was this more apparent than on Ancient Earth itself, the seat and capital of that ancient empire, and storehouse of its secrets.
For centuries such dread secrets slumbered; in forgotten bunkers and armoured vaults lay the masterless gene-looms and autosurgical chambers of the Dark Age of Technology, waiting for the ignorant and the power hungry to set them to work once more to bloody and ill-reckoned purposes.
The end results of their inevitable rediscovery was a world already plunged into violent anarchy and environmental collapse made infinitely worse by the abused powers of those lost gods and devils of ancient science.
These outcomes included genetically tailored warfare, desperate schemes at blood-bought immortality and power, and a hulking grotesquery of once-human shock troops driven into battle by the ambition of crazed warlords.
Mutation, monstrosity, pogrom, disease and death unimaginable were let loose by the warlords' lust for conquest; such was Terra's long night.
Warders of Rython
During the bloody years of the Unification Wars, when the Emperor launched His campaign to finally crush the last of the techno-barbarian warlords and tyrants that had held sway over Mankind during Old Night, and later, during the galaxy-wide campaign to reunite the disparate worlds of Mankind, the Emperor encountered many technological terrors, unholy constructs and vile entities, created by both human and xenos alike, as if they had been dredged up from pure nightmare.
The Emperor knew that such aberrant technology could not be allowed to fall into the hands of the unwary, nor could such foul aberrations be allowed to walk amongst the worlds of Mankind, for they would be humanity's inevitable downfall.
In His wisdom, the Emperor charged a detachment of His own Legio Custodes to act as both wardens and gaolers -- to hold eternal vigil over the horrors they had encountered -- and to lock them away deep beneath the massive Imperial Palace complex on Terra itself.
These items and captured entities would be sequestered to the Vaults of Rython, where they would be held within rune-locked portals, and warded by the power of sanctic circles -- forbidden to all save their eternal guardians, the so-called Warders of Rython and the Emperor Himself.
Ten millennia later, to the Shield Host of the Adeptus Custodes known as the Shadowkeepers falls the sacred duty of standing guard over the Dark Cells, a particularly sequestered and forbidden sub-division of the deep levels below the Imperial Palace complex on Terra, unto the end of time.
The Shadowkeepers hold the keys to the rune-locked portals hidden deep beneath the Imperial Palace. They alone know the ways by which the runic locks may be disengaged, the wards unbound and the sanctic circles breached. They alone know that these things must never happen, for the Dark Cells hold such horrors at bay that Mankind's sanity would not survive their release.
A full Shield Host is devoted to this grim responsibility, over a hundred Custodians patrolling the dark and silent corridors, vigilantly watching over the last terrors of Old Night. It is a task that would soon drive most men mad, for though neither sight nor sound can escape the forbidden cells, the air of those corridors is charged with dread.
A perpetual menace thickens the shadows and makes them crawl. Even the superhumans of the Adeptus Custodes are forever on edge in those dark oubliettes, for the sense of unspeakable threat never wanes. It is a testament to the discipline and spiritual fortitude of the Shadowkeepers that they stand their guard unflinching, sometimes for solar decades at a time.
The ranks of this Shield Host include many Custodian Wardens, whose oaths of protection help them to focus upon the task at hand to the exclusion of all else. The leaders of these forbidding sentries carry ancient weapons of mysterious provenance, their use intended as a last resort should anything ever break free from the Dark Cells.
For ten thousand standard years the Shadowkeepers have performed their duty, yet the opening of the Great Rift changed everything. With the power of Chaos spilling raw and seething into the spaces between the stars, new abominations have come to light. Worse still are the cells that stand suddenly empty, the entities and artefacts once contained within spirited away by some unholy force to curse the galaxy once more.
Fearing the consequences of such dread remnants of the Age of Strife falling into the wrong hands, the Shadowkeepers at last sent warriors out into the galaxy.
Not since the dark times of the Horus Heresy, when several sodalities of Shadowkeepers deployed as a part of the Prosperine Censure Host during the Burning of Prospero, has this Shield Host deployed en masse.
In this darkest of ages, these jailers must trammel that which should not be, slaughtering all who seek to impede them, before returning their foul prizes to the cells where they belong.
There have been many Custodes Shield-Captains charged with mastery of the Shadowkeepers. This appointment confers the title of "Lockwarden," a name that is borne in perpetuity and garners solemn respect from every other member of the Ten Thousand.
The Lockwarden must be the sternest of all guardians, the most unrelenting and alert gaoler on the face of Terra. Moreover, should any creature or relic escape the Dark Cells, or newly emerged threat need to be imprisoned therein, it is the duty of the Lockwarden to personally oversee the operation.
The current incumbent of this position is Shield-Captain Borsa Thursk, who has been Lockwarden for a Terran century and a half. He is a grim and frighteningly intense warrior whose utter fearlessness and steely vigilance make him ideal for his role.
It speaks volumes about the dire condition of the galaxy that Thursk left Terra but twice before the breaking of the storm, yet he has barely set foot there since the Great Rift yawned wide.
- The Burning of Prospero (004.M31) - Several sodalities were drawn from the so-called "Warders of the Vaults of Rython" which were deployed as a part of the Imperial Censure Host, Terran Contingent, serving as the reserve second wave behind the lead elements commanded by Captain-General Constatin Valdor during the assault on Prospero. The fact that these forces were deployed as a part of the Censure Force is itself something of a mystery, and one some have suggested speaks to a need to call upon reserves more desperate than was outwardly understood at the time. Vitally, the Warder Sodalities resisted the circling counterattacks made by the Thousand Sons forces during the Battle for Tizca's cataclysmic final hours, holding back their psychically-augmented onslaughts when the merely human troops defending the Imperium's supply lines and landing zones broke and were slaughtered around them.
- The Ominous Gift (Unknown Date.M33-M39) - At some point between the 33rd and the 39th Millennia, Halo-belt Augurs reveal the Space Hulk Ominous Gift advancing inexorably out of the dark void towards Terra. Using his status as a High Lord of Terra to overrule objections by the Imperial Navy, Captain-General Aesoth Koumadra orders a strike by several Shield Companies to gut the craft from the inside and ensure its corruption is wholly purged. Those outside the Adeptus Custodes do not understand the significance, but the attack is led by the Lockwarden of the Shadowkeepers and a band of his black-armoured comrades. The Ominous Gift is destroyed -- and the wider Imperium need never know any more than that.
- The Years of Madness (Unknown Date.M41) - Whispers engulf Terra, beginning with the disappearance of the notoriously conservative Captain-General Galahoth. Battling the stagnation of Galahoth's rule, the Adeptus Custodes find themselves facing a shocking increase in cult activity -- both heretical and xenophile -- throughout the Sol System. Doomsday sects trigger queue-wars between the pilgrim tribes within the Emperor's palace itself, and the Custodians are forced to exercise their authority in the bloodiest fights they have seen in centuries. Reports from the Dark Cells cite a growing sense of agitation amongst the hidden inmates, and numerous support Servitors have to be destroyed by the Shadowkeepers after they exhibit sudden, violent madness. Worse is to follow as possession is revealed amongst a sub-sect of the doomscryers of the Imperial Palace themselves, though not until the false predictions of the fallen psykers send Captain-General Launceddre to his death at the Battle of the Gilded Pyre. It is amidst this climate of spiralling paranoia and danger that Captain-General Trajann Valoris is elevated to command the Ten Thousand, and he wastes no time in taking steps to regain ironclad control of Terra's defences.
- Into Shadow (Unknown Date.M42) - Upon the direct orders of Trajann Valoris, a small, fast-moving force of Custodians makes haste for the ruined remains of Cadia. Details of their mission are suppressed, even amongst their comrades, but they are accompanied by a number of warriors drawn from the ranks of the Shadowkeepers.
- Codex: Adeptus Custodes (8th Edition), pp. 23, 31, 33, 49
- The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 269