This outworld, close to the Scarus Sector border, is unninhabited. However, it is famous for its sulphurous volcanic vents, through which, it is said, the voice of the warp whispers. It is said the skin of reality is thin there and one might hear the vibration of the immaterium, just out of reach. It is said that on Sleef, the warp whispers in the thunder of the volcanoes and can change the fate of those who hear it. The stones of Sleef are psychoreactive, making them powerful tools to be used in the divination school of lithomancy.

Sleef was the site of a battle between the elderly Inquisitor Katuwe Orne of the Ocularians and a necrophagic witch-cult that had travelled there after foretelling of the birth of a prophesied child. This so-called ‘child of midnight’ would be a true abomination, an alpha-plus level psyker with the power to enslave millions and carve an empire of its own out of the Imperium’s bloody carcass. Orne’s vision on Sleef has led him to spend his life’s work against this dark future.

Sleef was the planet where Inquisitor Joshua Starkh first met a man he only came to know as the Stranger, who threatened him and his cell. They chased him off-world, tracking him all over Calixis before cornering him on Baraspine.

In 596.M41, a schismatical rose from the Hadd depths infused with knowledge of forbidden Empyric technology. It sought to open the way to transcendence within the warp and claimed enough of the low Machine Temples of Hadd Nordus to work effectively in secret. The schismatical was only discovered before it achieved its aim because it sent echoes of itself within a Mechanicus vessel across the voids to Sleef, an act which led the Ordos Calixis back to its machine coven upon Hadd. The Mechanicus vessel was destroyed by the Battlefleet ships stationed there. An ensuing moral threat struck at Battlefleet-pledged tech-adepts, and warp-tainted Tech-priests were later destroyed upon Sleef’s volcanic surface.

Some survived, however. For centuries, a fragmentary schismatical has driven a handful of age-maddened tech-adepts and damaged servitors in a toil to build a mighty warp-machine. It aims to penetrate the barrier between Materium and Empyrean and must be stopped.