"The Warp is a child's play pen compared to that place; mark my words, you cross the Stygian Break and you had best be ready to die."
- —Callat Glass, void-master of the Mark of Perdition
The Black Reef is an astronomical phenomenon of intense, unnatural gravity found within the Jericho Reach.
The Reach is littered with celestial perils from the Slinnar Deep Gulfs to the Hadex Anomaly, a combination of stellar debris and void-born chaos so common in the far galactic east.
Among these wild zones, the Black Reef stands as a briar patch of gravitational madness stretching across a dozen light years and effectively closing off an entire area of space.
Located at the extreme edge of the Canis Salient on the coreward rim of the sector, it has for solar decades been a natural shield against the Achilus Crusade's advance, providing the outposts of the T'au Empire in the Reach with a solid edifice against attack.
Bordered by the T'au worlds of Tsua'Malor, Krrk'tikit and Iphigenia, the Black Reef is practically impassable to voidfaring ships, its gravity storms reaching into the Warp itself, fouling Warp routes and disturbing the Immaterium, forcing vessels to return to realspace and face the anomaly's brutal power.
The T'au have made extensive use of this barrier to shield their worlds from assault and have created their defensive line to either side, confident that no Imperial battlegroup could hope to cross it alive.
The Black Reef is far more than a mere celestial hazard however, and over the years both the Humans and the T'au have made attempts to determine its purpose and perhaps what lies at its core.
It is known to both sides that it covers a number of ancient systems and perhaps hides the remains of some long lost civilisation, one that predates both the T'au and the Imperial presence in the sector. The laws of physics however break down as soon as a vessel enters the Reef and gravity loses all reason as it pulls, pushes and crushes areas of space seemingly at random.
Entire worlds are flung across the void, torn away from their stars by these random forces and imperiling all in their path.
Those few intrepid explorers that have penetrated deep into the Reef (and returned to pass on their findings) tell wild and conflicting tales of their experiences, though all seem to agree there is something hidden deep within its core, something big...
The Gravity Storm
The Black Reef is a gravitational anomaly which follows none of the natural laws of the universe. Vessels crossing into the Reef, across what has become known as the Stygian Break, are immediately assailed by gravitic eddies and tides.
These ripples across the void are practically impossible to detect or predict and any one can smash a ship to fragments. Even those vessels fortunate enough to avoid the worst of the gravity storm will likely be thrown wildly off course by the constant energy fluxes and shifting nature of the void.
This problem is compounded by the fact that the gravity storm does not just affect ships crossing the Reef, but all celestial objects and bodies within it, making it virtually impossible to take an accurate bearing as planets, gas clouds and even the stars themselves move erratically across the sky.
Gravity swells and navigational mayhem are not the only perils of the Black Reef, however, as it is filled with the detritus of dozens of worlds and the wreckage of thousands of vessels.
Given the vast size of the Reef, more than one captain has scoffed at the notion of "running into" such stellar flotsam and jetsam, and under normal circumstances it is a simple matter for a vessel to easily avoid such perils when plying the void.
However another astronomical quirk of the Black Reef is that the same gravitational ripples which can rip a ship apart seem to cause all kinds of mass to be drawn together. This means wrecks, asteroids and rogue planets will all seek out ships trying to cross the Reef, changing course and accelerating as if drawn in by some potent magnetic force.
Often a craft will have only moments to detect, avoid or destroy such objects as they appear, before they crash into the hull with massive destructive force.
The Black Heart
Several attempts by Adeptus Mechanicus Explorator fleets have been made to navigate the Black Reef, though to date all have met with failure. Lord Commander Sebiascor Ebongrave himself has also launched numerous scouting parties into the Reef in the hopes of somehow flanking the T'au positions and gaining a decisive military victory.
These too have met only with failure, many of them disappearing completely, leading the paranoid Ebongrave to suspect treachery. A few Imperial explorers have managed to delve deep into the Reef and return, among them the famed Rogue Trader Telas Kai and his ship the Ancient Fable.
What he has passed on seems hard to believe, though if true could turn around the fortunes of the Canis Salient in a single stroke. Telas asserts that beyond the chaos of the Reef's outer regions lies a dark centre and a macro-system of dozens of worlds from dozens of systems, all drawn into the orbit of a single, massive, star-sized object.
As with the rest of the Reef, this macro-system, dubbed the "Black Heart" by Kai, is a tangled web of gravity fields and unpredictable orbits.
Kai tells how he explored several of the worlds and discovered the remains of an ancient, highly advanced xenos civilisation long turned to dust. He also encountered primitive reptilian aliens, which he deduced to be the descendants of the race which once dominated these worlds and this region of space.
The aliens, which called themselves the Syrshin, roam the worlds of the Black Heart, defending the ruins and warring among themselves. Without the technology to cross the void, Kai discovered they could use naturally occurring gravity wells, vast plumes of air and water millions of kilometres in length, which would periodically link the planets and allow the Syrshin to literally "fly" between worlds.
Cobbling together xenos writing from the walls of the ruins, and secrets pulled from the minds of the Syrshin by his psykers, Telas came to the conclusion that they were waiting for some powerful enemy which they believed would soon return, rising from a millennia-long slumber, once more to enslave them as it had so long ago.
Of more interest to Kai than the ghost stories of the Syrshin was what he discovered about the great mass at the centre of the Black Heart. Though he was not able to get closer than a billion kilometres from it, he believes it is in fact a vast engine built by the lost Syrshin civilisation to protect them from the ancient enemy and is the cause of the Reef itself.
Telas also suspected that while no ship could approach, it might be possible to travel "on foot" using the gravity wells to skip between worlds. Unfortunately for Telas, the shifting gravity tides forced him to leave the Black Heart before he could put his theory to the test.
Thus far he has not been able to find the Heart again, though Ebongrave has become interested in his tales and the possibility of harnessing this Syrshin Singularity Engine. He is eagerly funding a return trip for the Rogue Trader.
The T'au and the Black Reef
The Humans are not the only ones to explore the Black Reef and the T'au too have been sending scouts into its gravity-ravaged depths. Less willing to needlessly sacrifice their warriors, the T'au have employed those aliens from their empire with an affinity for the void and a skill at space faring, such as the ethereal Ji'atrix and the void-dwelling Nicassar.
More successfully than the Imperium, the T'au have mapped a number of routes through the edges of the Reef, at least on their side of the anomaly, allowing their voidships more tactical flexibility than those of their enemies and also allowing them to lay deadly ambushes within the edge of the Stygian Break.
The T'au also enjoy a rare speed advantage over the Imperium within the Black Reef as their unusual method of near-Warp travel works more effectively than completely entering the Warp and is less prone to disruption by the gravity storm.
The T'au have long been aware of the Syrshin and their lost empire, from artefacts gathered from the fragmented worlds of the Reef. Aun'El Basel'kyth himself has taken an interest in understanding the empire which once dominated this area of space.
The senior T'au researcher's interest in the Black Reef and the amount of T'au resources allocated to its secrets has increased in recent times since T'au spies learned of Kai's discovery of the Black Heart.
While Aun'El Basel'kyth does not fully believe the wild stories of the Rogue Trader, his research so far makes him believe that such a thing as the Singularity Engine could exist.
While the T'au high command in the Jericho Reach is generally skeptical about the existence of such a device, they have nevertheless decided to allow Aun'El Basel'kyth to make a serious attempt at reaching the Black Heart before the Imperium and harnessing the power of the Syrshin Singularity Engine before Telas Kai and his crew can find its location again.
- Deathwatch: The Achilus Assault (RPG), pp. 129-130