"It might have been madness on Lord Ebongrave's part that quarantined this cursed world, but having seen it for myself and knowing what lurks at the bottom of that ocean I'll be damned if he wasn't right."
- —Captain Loris, Drusian 54th Forward Regiment
Rheelas is an Imperial Mining World that lies within the Jericho Reach's Arkalas Cluster, just beyond the edge of the Iron Collar and the Well of Night. Its citizens rejoiced when the Achilus Crusade stormed into the Reach and welcomed them back into the Imperium as part of the crusade's Canis Salient.
It was a short-lived celebration, however, as even as the people of Rheelas renewed their oaths to the God-Emperor and prepared to support the crusade with their manpower and resources, the shadow of suspicion and doubt fell upon them from the salient's supreme commander, Lord Commander Sebiascor Ebongrave.
Paranoid of T'au sympathisers and seditionists seeking to seduce Humanity into service to the Greater Good, Ebongrave first sanctioned the planet and then declared it fully quarantined, blockading its spaceports and closing off its system to all trade. It was such a sudden and absolute order that it even gave the crusade's Warmaster pause, though in the end Solomon Tetrarchus chose not to counter Ebongrave and to instead accept the commander's fervent belief that the T'au would use Rheelas as a second front against his forces.
For years, the world of Rheelas has languished under the blockade, sinking deeper and deeper into ruin and chaos. Its once great mining cities, seaborne hive-like structures (the continents of Rheelas being far too unstable for permanent settlement), have begun to rust and rot with no market for the ores they once drilled from the sea floor.
During much of this time Ebongrave gave no mind to the people of Rheelas, considering them Traitors and xenos cultists of no concern, focusing instead on the expanding war with the T'au around the Black Reef. It was only when faced with reports of a full-blown uprising and the capture of several orbital installations that Ebongrave grudgingly diverted resources to deal with the world.
In the ensuing rebellion, a number of Space Marine Chapters were also drawn into the fighting on Rheelas, though for different reasons. As it happened, the majority of the population was not involved in the rebellion, and it was merely part of a larger plan by the forces of Chaos active in the Reach to extend their front and sow discord behind the Imperial lines.
In the aftermath of the rebellion, several of the great seaborne hive cities were sunk, among them Hellsmark, a centre for the fighting. The news of the uprising and its outcome drew some attention from the Dead Cabal, as the world had always been of interest to them, rumours of ancient xenos scattered throughout its history.
It was only when survivors of Hellsmark dived down to the sunken city to scavenge mining equipment that they discovered something unusual, something which drew the attention of the Dead Cabal.
It appeared to the miners that when the city struck the sea floor it broke through into a series of underwater caverns and tunnels, the ruined sections of the city merging with the ruins and draining many of them dry for the first time in thousands of Terran years.
Those few miners who managed to return to the surface reported things sleeping in the caves, and machines like they had never seen before. Taking a personal interest, Ordo Xenos Inquisitor Hoath and his retinue went to Rheelas to investigate, as the Inquisitor had already been in the region dealing with T'au seditionists.
This was only a few cycles ago, and Hoath's final message that he had descended to the city and discovered something troubling reached the Deathwatch and the Dead Cabal.
This is the task given to the Deathwatch: to reach Rheelas, cross the blockade and find the location of the sunken city. Then they must travel down to the sea floor kilometres below the surface and find the Inquisitor and just what it was he discovered, before attempting to contain or deal with it.
- Deathwatch: The Outer Reach (RPG), pg. 41